Why do cats sometimes put their but up in the air when you pet them?
Cats do that as it is part of their character to show that they appreciate the petting. It is a basic reaction to the cats, as it is pleasurable for them and to encourage you to rub them some more.
Because they want you to know that it feels good when you do that and they want you to keep doing it.
They do that because they show that they love whatever your doing or just to show you that they are getting ready to go into heat or is in heat