I've had this happen numerous times while working a flight. Of course, we'd get up to close the bin as soon as it was safe to get out of the jumpseat, but FAs should not be up at all when it's too bumpy (btw if they get injurred while being up when they shouldn't, their workers comp claim gets denied). The training is such that you need to protect yourself if you're going to be any good for protecting the passengers in an emergency.

Particularly during turbulent takeoffs and climbs, it is usually better to remain seated with your seatbelt fastened. Once you get up, now there are 2 "things" that can fall down and hit another passenger: The bag and you.

(On a side note, this is why it is so annoying for me as a customer to see others get up when it's bumpy and the seatbelt sign is illuminated - if they fall over that sucks for them but when they fall on another passenger it's so unfair to that person who was doing what they were supposed to be doing).

Probably the best thing you could have done is tried to get the attendtion of the person who was in danger so they could keep an eye on it and be ready to fend off the bag.