This would happen if something clogged the drain pipe after the intersection of the bathtub drain and floor drain, so that going up the floor drain became the e

This would happen if something clogged the drain pipe after the intersection of the bathtub drain and floor drain, so that going up the floor drain became the easiest path. Someone before you may have thrown away something that clogged the pipe, but sometimes very hard water, a lot of time and no maintenance can also create limestone deposits that effectively shrink the pipe.