My two do it because they are hungry. They like to wake up early to poop if they don't go the right before too, but 9 out of 10 times they go right to their food bowls at dawn and start flipping them over and kicking them across the room rather than going over and standing by the door, which is why I know their tummies and not their bladders are waking them up
I've started feeding them later in the day and giving them 1/2 meal for lunch and that seems to help. We've gone from 5am to 6:30, which is much better. I try to not feed them before 7 because then they want "dinner" at 2:00 so if they dance on my head at 5am I take them out to pee and give them a decent sized cookie and we all go back to bed for an hour or two. Once they get a potty break and a cookie they seem content to snuggle back in.
