Why baths block
As you may know, using a bath that has a blockage is really only making the blockage bigger. Bath blockages are mainly hair and everything else that gets sucked down the plug hole, bound together with soap scum. Continuing to use a bath that's already partly blocked just adds more material to the existing blockage.
Hair tangles together at the U-bend trap directly below the plug hole. Soap scum — particularly from bar soaps that contain fats — binds the hair into a solid mat. After enough time, water can't get past, and that's when the bath starts holding water.