Turbulence
OzExpat is spot on with his comments here.
Any decent pilot will try to warn passengers before hitting turbulence, and of course, there are insurance implications here, but clear air and un-forecast turbulence can hit at any time, and there's no way of knowing how rough or how prolonged it 's going to be, hence the message at the beginning of flights to advise passengers to remain seated at all times with seatbelt fastened.
Ideally, the seatbelt sign should be switched on at all times, but passengers (especially on long flights) have to be allowed to move around, for using the toilets etc.
I too have found myself in sudden unexpected turbulence and had my hands full, preventing me from making an announcement to passengers.