Thumbless grip.
Aye, if the spotters were there and they missed it, you have to think he dropped it when he wasn't in the lift portion, which does suggest thumbless grip.
It also suggests maybe a combination of bent bar that pulled out of his hands and/or his incapacity to handle the weight (ie, not controlled reps).
Whenever a thread like this appears, the anti-benchers come out in droves, which is fair enough but the truth is that benching isn't bad if you are built to do it and if you've been lifting for even a few years, you will know whether you are.
I am built to do so and I handled 315+ every week last year, often going for big singles for me (plenty of failures).. feel fine, zero injuries, nothing. Never felt unsafe. Failed 4 plates a couple of times, no problem. Have been doing that for years, now.
Like I say, you either have it or you do not. I do not have steroid inflated strength, though, if that counts. Not punching above my weight.