Basically he and the manufacturer were able to conclude that some A-hole refilled some of the rounds with pistol powder instead of rifle powder, then sold it on as standard surplus. Pistol powder burns hotter and faster than rifle rounds. After doing the math and more testing, Mark Serbu thinks the faulty round put out something like 200,000 psi.

The most wholesome thing about this incident was all of his guntuber friends made videos in his style for him to reacted and upload them so his channel would had contents.

Nutshell version of the after accident investigation done by Scott and Mark Serbu, the creator of the gun, for those interested in some additional context:
-The ammo was likely reloaded ammunition, as the ballistics of the different rounds were very inconsistent.
-The round that caused the accident was likely filled with either pistol powder or a mixture of pistol and rifle powder. Pistol powder burns hotter and faster than rifle powder, causing more PSI of pressure for the same weight of powder.
-Serbu reevaluated the estimate for the PSI for the accident after this video. When he first heard about it from Scott, he misunderstood and thought just the cap blew off. The force needed for a rapid disassembly like that is closer to 190k PSI. For reference, a standard 50 caliber bullet is around 65k PSI. Scott tested it himself in another video after Serbu sent him another of the same rifle for testing and more or less confirmed that conclusion.
-Scott and Serbu both stated that the guy who sold him the ammo is likely not to blame for the improper reloading of the round. As Scott mentioned in the video, these rounds have been sold and resold for a long time - the last time these rounds were used by the US military was Operation Desert Storm in 1990 and the accident was in 2021. Anyone who bought the rounds at any point from then to Scott could have reloaded them improperly and no one would have known until it went off.
-Scott Sr. and Scott's training are basically the only reason he's still alive. Both are former police and they both had first aid training. Not enough to fix it on their own, but enough to keep him alive until he can get to someone who can.
-Most importantly, Scott is fine and doing better every day since. He mentioned in later videos that the only permanent issues he got from the accident was numbness in the back of his mouth where the surgeon cut in, the impacted eye doesn’t fully dilate or constrict, and the scars from the injury and the surgery. He also shared that after his chest healed, he started bench pressing again and is benching more than he ever has now. He openly talked about the mental scars in a later video but while he still has some flashbacks to that day, he’s not letting that fear control him and he knows it was a freak accident. That being said, he is a lot more careful about ammo now.
-Scott is now focused on two things - being absolutely jacked and getting the biggest, hardest recoiling guns he can get his hands on.
-Scott also put out a kid’s book about being brave after an accident, sharing the coping methods he’s learned since the accident.

My dad, being an avid hiker and camper, had this wonderful T-shirt that said "You could fall off a cliff and die. You could get mauled by a bear and die. You could fall into a Class V rapid and die. ... Or you could stay home, fall off the couch and die."
I always remember that when people talk about "Oh I don't do these dangerous things because I might die!"  The truth is, you're never that far from death, even when you're 'safe'.  I don't ever want to become so intent on surviving, that I forget to actually live.
There's another quote: "All men die, but not all truly live."  Scott embodies this well.

Man literally hit a Nat 20 on his Death Save and then Nat 20 on recovery.

I'd like to mention that Scott Sr (the dad) has a military medic background. This is what gave him the skills and knowledge to act as he did that day. Which is also a double-edged sword as he also had the knowledge to know how badly his son was injured, and what a low chance of survival he may have had.

I have much respect for Scott Sr.

As someone with MS and has been in a car accident which should have killed me, all hospital worker should be paid extremely well

now that you saw this, there's now also the opportunity to watch the experiment he did where he used all the leftover SLAP rounds to try and recreate his accident, thankfully using a ballistic dummy head instead of doing it himself

Tampons are NOT great for gunshot wounds. That is a dangerous myth.

As far as Scott’s mental state, he probably has a little bit of PTSD. He has admitted that his neck starts to itch when he shoots large caliber guns. But, he carries on!

The rounds he was using are a very old type that isn't professionally made anymore. They are called SLAP or Sabot Light Armor Penetrator rounds and they have been exchanging hands over the years. At some point someone loaded them with the finer grain gunpowder used in handguns which burns hotter and faster than the kind that long barrel rifles are supposed to use. This resulted in extremely high pressure inside the barrel resulting in a catastrophic failure. The pressure inside has been estimate by the manufacturer of the rifle to be around 200k psi. Which is about 4 times what the rifle is rated for.

So, gosh I remember when this happened. So his friends have talked about hearing about the incident. Donut when he got the call all he was told was “Scott had an accident”. And he was the one to then tell Brandon. But the way donut said the voice of Scott’s father was just he could tell the gravity of the situation with the tone used. Scott is an unbelievably lucky and beautiful man, to survive with sheer willpower is incredible.

So, as others have said, the bullet in question was theorized to have been loaded with a mixture of pistol powder and rifle powder. But the significance is that because of the difference in barrel length, there's not a lot of time for the pressure to build up in order to propel the bullet compared to a rifle. So to compensate for that, pistol powder burns much, MUCH faster than rifle powder.
Also, the 85,000 psi that Mark Serbu gave was based on hearing that the threads had been sheared off. Upon ACTUALLY seeing what happened to the rifle, Serbu updated the estimate to around 190,000 psi

I respect the hell out of his courage and mental fortitude to be able to shoot that pistol at the end so soon after such a terrible accident.

One thing I love about this is that in the immediate aftermath, several other gun tubers made videos in Scott's style to post on his channel to keep it going and sold special merchandise to help him pay his bills.
Another interesting video is by Ballistic High-Speed where they had an RPG explode on them while they were using it.

On the medical side, the only thing Scott was wrong about is the direction of the jugular vein, and I dont blame him for it, but a quick correction:
You have four jugular veins, a right and left external jugular and a right and left internal jugular. Any "vein," returns to the heart, meaning that the jugular returns blood from the head and brain to the heart. Arteries supply blood from the heart to the body, the prominent ones in your neck are the left and right carotid arteries. This is the artery you feel a pulse from when you feel a neck for a pulse.

13:03 I’ve made this comment on so many reactions to his video. You are the first to ever say it in a video. I still can’t imagine what was going through his dads mind from the moment it ended to when he was discahrged

23:10 When I understood the weakness of my flesh, it disgusted me...

You’ve unlocked: New Level of Respect for Scott

The bullet was loaded with pistol powder, likely not by a manufacturer but someone who decided to reload the round. They either accidentally mixed up the powders, or they were leaving a trap for someone. He tested the rest of the rounds and they behaved very inconsistently, but most of them had severe overpressure, although they didn't destroy the gun. The round that blew his gun went off with nearly 200,000 PSI of pressure. He was both extremely unlucky to have the ear of the rifle nearly kill him, but also lucky the other one didn't go into his head. And at the end, him shooting a gun is him saying he didn't hear a bell, and he's not going to let that stop him...and he's not stopped in the five years since.