I agree with other replies here with one addition. People who get into grad school are generally the high achievers from their high school and undergrad programs, so they are used to being the only or one of few star students. In PhD programs everyone is a star student, so it can be a bit jarring to folks used to being head of the class when they suddenly aren’t.

It can cause a bit of jealousy and competitive thinking but most of the time students get past it and focus simply on enduring the gauntlet.