Why baths block
Most bath blockages are hair, bound together with soap scum. The hair tangles in the trap directly below the plug hole, and soap fats then glue it into a solid mat. Because of that, water slows down and the bath starts to hold water.
Using a half-blocked bath makes things worse. Each wash pushes more hair and scum onto the mat, so a slow bath is best dealt with early. If the bath keeps backing up, our guide to why a bathtub drain backs up explains the common causes.