I've had a steel seat post do this because it and the seat post clamp had been tightened beyond anything it ever needed to be and the post and seat tube were deformed. I can't help but think this would happen to aluminum as well. At any rate, I would get the seat back on the post as you don't want the post to fall into the seat tube. And if the post is deformed, I would drill a hole horizontally through it and put a screwdriver or rod through it to get some leverage on and then replace the post. HTH.