When cats lift their backs when petted, it is a sign of enjoyment and comfort. This behavior, known as "elevator butt", allows cats to properly position themselves to receive affection and interact with their owners. By arching their backs, cats are able to signal that they are in a good mood and are enjoying the attention being given to them. This action also helps cats stretch their muscles and show trust towards their human companions.

Additionally, cats may lift their backs to better control the pressure and positioning of the petting they are receiving. Overall, this behavior is a positive and normal response from cats to the physical touch and interaction they receive from their owners.