I personally just think cats can't stand a closed door.
When I take a bath and have the bathroom door closed tight, mine will mew and mew to be let in. So I get out, drip all over the floor, let her in. A few minutes later, she's mewing to get back out.
I keep the bathroom door closed so she's not in and out and in and out; esp. in the winter when I have the space heater running in the bathroom (our house is from the 40s and the only heat vents are in the kitchen, living room, and downstairs bedroom) and i don't want her opening the door and letting the heat out. Lately I've just taken to pushing the door to so it's closed but not latched, and keeping this big dowel rod we bought once for a bathroom project and never used, by the tub, and it's long enough to shove the door shut from the tub when she comes in. Of course, she'll shove the door open and come in, but as soon as I move to close it behind her, she freaks out and runs back out.