I wonder if it’s the cantilever action of a bike rack which puts more pressure on the ball connection with a turning motion were as as trailer/caravan just applies a downward force close to the mounting on the vehicle
Yes it is. With a trailer the load is above the ball and with a bike rack its some distance behind the ball. It the reason the heavy bikes go on first to reduce the bending moment. Having said all this in reality there will be a substantial factor of safety in the design and covering the fact someone somewhere will have a rack miles from the ball and the heavy bike is far away.