My cat sometimes rubs his but on little trees and fallen branches?

My cat is a country cat. Kills birds, rats just like normal cat. When I'm outside sometimes we walk into the woods and 1st he stiffs the tree the turns around and rubs his but of a certain spot. He's tail also shakes in a weird way. The answer might be simple like maybe marking his territorie or something but I just want to make sure. Thx :)