Lol, Matt Cutts answerd the following question on the webmasterchannel on YouTube. "Why do cats bums go up when you stroke them?"

http://www.youtube.com/watch?v=xzhXif9jRWM

Or rather, he didn't answer the question.

Elementary, my dear Cutts, cats do it to offer some fur resistance/ bigger bang per stroke ratio, and signal the willingness to mate. You really should consult some 12 year old cat fancier prior to attempting to answer such deep-knowledge questions as that.