You will sometimes bounce off the trunk a bit. It is so small an impact that it is nothing at all.I am envisioning if you are holding yourself off the tree with your leg extended and you suddenly slip you could swing beside or around the tree trunk. What would concern me most is the pendulum swing back towards the trunk. But as I consider what you said about the tree leaning I can see my scenario is probably an overblown hypothetical, especially if you rappel slowly and maintain good contact with the tree. You probably can’t get more than a few feet off the trunk at any given time to gain that much momentum.