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.
People with autism sometimes have a hard time paying attention, when someone calls out their name especially; this is because once they’re highly focused on something interesting, it’s hard to draw their attention back.
Timmy doesn’t respond very well to yelling, and anyone who knows an autistic person needs to learn to be patient. It doesn’t hurt to call their name once again in a calm manner, they’ll listen eventually!
They’re very common around the mind of autism, especially being triggered by minor things (noise, taste, or change of routine). An emotional breakdown that involves screaming and sometimes hitting. It’s not easy calming them down, especially when they’re still a kid.
In this case, Tim has a phobia for balloons, the loud pop is very unsettling especially against the ear. (I hate it a lot too :|) He’s just really scared, unfortunately others don’t understand just as much as his brothers and mom.
Anything that may calm an autistic person could be a certain texture they like, an interest they love to distract them from their meltdown. Tim loves anything nostalgic and fluffy, like Mr. Bunnington!