Here's a pic of the separation:
The Acela coaches aren't designed to be "uncoupled" as are normal coaches.
Instead, they have a solid "drawbar" between them.
Looks like it rests in a large "pin" bolted to the bottom of the end of the car.
I'll guess that something around that large pin "gave way".
Maybe the bolts that secured it to the bottom of the car.
Or perhaps there was some kind of "retaining" assembly -under- the drawbar that broke.
I had this happen back around 1985 when I was getting qualified on the Shore Line between New Haven and Boston.
I was on train 179 (not running, just riding in the fireman's seat). Coming to Guilford, we hit something on the track, might have been a piece of iron put there by kids.
Anyway, something like that isn't enough to derail the train, but it starts banging around underneath. It must have caught an uncoupling lever.
We were going around a curve to the left (fireman's seat is on the left side of the engine).
Train goes into emergency at 90mph.
I looked out the window, and there were only two cars back there instead of the four we left Boston with!
They had stopped a lot more quickly (without the weight of the engine), and were back there out-of-sight.
We backed up, were able to couple them up, reconnect the lighting cables, and got back to New Haven.
But it wouldn't have been good if anyone had been moving between the cars when the train broke apart!
(BTW, train separations can be much more common in freight, usually from a broken knuckle due to all the weight behind it)