I looked up 50 BMG SLAP rounds on a military tech site.

I had never heard of them.

Caliber .50 Saboted Light Armor Penetrator (SLAP)

These consisted of a smaller-than-caliber tungsten bullet in a plastic sabot that made a round for a 50 cal gun.

The plastic sabot would fall away after leaving the muzzle leaving a very fast moving small diameter bullet -- nearly 4000 fps.

At the time this cartridge was developed, the Marines also tried to make a 7.62 x 51 version.

The 50 cal version worked, but the 7.62 version did not. In the 30 cal version, the plastic sabot sometimes explosively disintegrated while still inside the barrel -- resulting in the bullet penetrated the side of the barrel destroying the gun.

So this makes me thing something like this happened.

The old plastic in the sabot, probably dried out and became brittle over 25 years.

The sabots were probably partially disintegrating in the first three shots. This probably left the smaller than 50 caliber bullet exiting the barrel out of alignment and maybe tumbling. That was likely the cause of the poor accuracy. It looked like the third shot hit a couple feet from point of aim.

In the final shot, perhaps the sabot explosively self destructed throwing the bullet off course and jamming it in the barrel resulting in the kaboom.

Sort of like the container ship in the Suez.

Just a theory.