A lot of autistic people have repetitive/ritualistic behaviors, and every one of them has different mannerisms compared to the other. Some repetitive behaviors can be harmless, some can be a little severe that it causes them to hurt themselves (such as scratching at skin). You can see that this is not all of Tim’s behaviors, mostly main ones; some that you have seen him do in the previous comics if you look back.
You can see in panel 4 there’s another hinted mannerism where he tugs his tie too hard at times when he’s very uneasy, he could accidentally choke himself a little which is why Evan makes sure to stop him right away. Some repetition can be considered as stimming depending on the person with autism; something that helps them to relax but that will be explained later in another comic.