As long as the "please keep your seatbelts closed during flight" is just a friendly recommendation, some people will invariably unfasten theirs as soon as the sign goes off (some even earlier).
The problem with CAT of course is, that there is usually no warning. The phenomenon is known, and that is the very reason that it is recommended to keep buckled up, unless you have business wandering around the cabin, as part of your job (Cabin Crew), or for personal hygiene.
The weather radar cannot pick up CAT, since EM radiation only reflects off water droplets, and not clear air. So probably the seat-belt sign was off, people were wandering around or just feeling more comfortable without the belt, galley trolleys may have been in the aisles; lots of potential for injuries.
Originally Posted by Little_Red_Hat
SB sign on= pax & cc seated, no exceptions.
Generally true, of course. The exception being that, prior to takeoff and landing, and if there is advance warning of turbulence, the sign goes on, and CC check that every passenger is buckled in, before strapping themselves down.