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Basically too much powder for the bullet weight or just too much powder. His previous SLAP rounds were wonky so it could have been incorrectly reloaded or the powder degraded.
Rex of Tiborsaurus Rex, a very knowledgeable shooter with a Youtube channel, theorized that it could have been caused by the muzzle brake.

It's cautioned not to shoot SLAP rounds in a rifle with a muzzle brake or at least a specially designed brake.

Some of the people who regularly post of Class III firearms forums have been commenting about this, mostly leaning in the direction of bad/degraded powder charge. Those sort of exotic .50BMG rounds cost $$$, so there's a strong possibility that someone in the decades-long chain of ownership of that cartridge stored it in a very hot environment - or worse, they may have dropped the round into a tumbler to polish away tarnish on the brass case. That does ugly things to the gunpowder inside the case, breaking it down into finer granules nd increasing its burn rate beyond anyone's ability to estimate.

The smart thing with those rounds is to safely remove the projectile and dump the old powder, reloading it with a known charge of fresh gunpowder.

One other guess that I saw mentioned: early failure of the sabot, while still in the bore. A sabot strike on a muzzle brake would be very dangerous, but it would happen at the muzzle end of the gun, just as the projectile clears the muzzle crown - no breech overpressure. However, if the sabot crumpled instantly and allowed the sub-caliber penetrator to yaw in the bore, rather than traveling straight ahead... yeah, that could act like a bore obstruction long enough for the breech to blow apart.

It'll be interesting to see what condition the barrel is in.