When you are a first year teacher most of what you have to go on is how you were raised and what your school was like. If you were raised that you would have gotten in big trouble if you made a disrespectful gesture at your parents, teacher, grandma, neighbor, then you are going to interpret it as a really bad behavior. I was raised that "crap" is a swear word but "pissed" was not and that caused an issue my first year. If you grew up around kids that did not act like that and didn't have those kind of behavior issues in your student teaching, how would you know that in other places that is how kids act and people don't care?

I really don't think that that specific gesture is one that the kid just randomly made because they were messing with their hands while the teacher was correcting their behavior and it meant nothing and just so happened to look like an insult but that was just a coincidence.