[laughing_man]

The leading theory, from what I can tell, is slosh. The flip on separation is relatively violent, and while there's not much fuel left compared to the amount at launch, it's still heavy enough to do damage.

[staplung]

The cast I was watching said that during the flip and boost-back the booster is pulling 11 Gs. That's a pretty hard hitting water-hammer for sure.

[smallmancontrov]

Oof, yeah, one good slosh at 11G is probably all it takes. On the bright side, there is probably a trajectory that works and I wish them luck in finding it!