Muzzle rise with machine pistols has always been an issue, especially with the high cyclic rate. As Serellan stated, the Beretta 93r (full auto 92) has a built in forward grip that swings down. Likewise the number of MACs that were sold with straps hanging from the front to use as a grip (although that was more to do with civy laws and a non-machinegun pistol with a forward grip is a no-no (it then become an AOW classified weapon under the NFA). Just look how high the thing points when he held long enough for a three round burst.
Anyway, forward grips aren't the problem. Just using them improperly is. It's kind of odd how it came back down on him though, recoil should have lifted the gun up away from the now unsupported hand. I think after it let go he just held in the trigger too long while moving the gun back down into position... and the unsupported hand stayed still ending up in the muzzle sweep.