After any activity that would expand the anal cavity, such as anal sex, if on pushees his or her sphincter, part of the anal tissue will come out of the rectum. It is given this name because it when extended it looks like a tulip.
"You should have seen the tulips on Nate after last night."
Ass Tulip is a slang term used to refer to a condition where the rectum is pulled during anal sex due to a variety of factors including insufficient lubrication and a larger than average diameter penis.
After this Dutch chick begged me do her in the ass, she had a pink sock hanging from her ass that resembled an Ass Tulip. Can you imagine a whole bouquet of pink ass tulips in a pretty floral arrangement for Mother's Day or something like that?