When passing another rider, or when being passed, one bike will often animate in annoying jumps. It looks like one rider is hitting the other with his shoulder.
Here’s an example from a Zwift Insider video: https://youtu.be/u3Thm96_-IU?t=33
I have seen it much worse - to the point where rather than draft with another rider, I have stopped pedaling to let the other rider get away just to get the immersion-breaking animation to stop. I also don’t want to ruin some other zwifter’s workout just because we’re riding at the same speed.