Why? That all depends if the turbulence was expected. No reason to be paranoid like the Americans and leave the seat belt sign on for the duration of the flight. That's even more counter productive. Most carriers clearly state that "turbulence can occur at any time and seat belts should be fastened when in your seat even if the seat belt sign is switched off".
Clearly the crux of the problem.
So where's the solution?
One can not expect the uninformed and ignorant passengers to solve such a problem on their own.
Is there a reasonable chance that some will respond to video screen showing passengers bouncing off ceilings during a quiet time while dozing?
Or should the extra attention be for the crew to anticipate high risk encounters over a portion of the flight?