why might the pineapple on the bottom of the pineapple-upside-down-cake float into the batter?
There are two reasons I can think of to cause this problem. If the batters were not mixed up in one batch one might have been a little too wet and given room for movement. Alternately, if they were baked together in the oven on different shelves the temperatures and the direct heat (top or bottom) could be different for each one.

Good luck on the next batch.