The near certainty that if you don't stand right outside the only stall in the bathroom, waiting for its occupant to finish, someone will slip in ahead of you once it's vacated. This is exacerbated by the equal certainty that your date will conclude you've slipped out the back door of the restaurant if you don't return to the table immediately and explain what's happening. It's a good reason to take a cell phone to the bathroom.
So this guy is taking forever to leave the stall, two other people are lined up outside the men's room, and my date calls my cell phone to say she's sure I have commitment issues. I tell her I'm on stall waiting, and I'll be back just as soon as possible.