Power asymmetry and also group pressure. There is still plenty of tacit pressure to share despite the "optional" caveat. Someone who says directly "I don't like this, no thank you" even in very deferential words will garner some degree of negativity, either from the manager or the group. Same for someone who shares too much and winds up sobbing, or someone who shares a vulnerability inconsequential in comparison to what others share.
I don't even know how I would react if i was caught off-guard with a stupid question like that out of the blue with everyone watching. On a good day, I would probably laugh it off and say something obviously idiotic like "my vulnerability is that i work too hard" to indicate that I don't find this amusing. On a bad day, caught off-guard, I might actually take the bait, "become vulnerable" and blurt out something far more stupid in front of everybody.
