Gatekeeping is kind of an easy behavior to understand. Nobody likes it when the things they spent their lives studying/engaging with gets "taken" or "co-opted" by others that haven't put in the same work as you. Even the most level-headed nerd can snap when things get too trendy and threatens the thing you once thought made you singular and special. This happens all the time, sometimes people feel protective of the things they love and that can lead to some jerky behavior. When it goes too far you get some truly cringe-worthy opinions.
Here's a few times the gatekeepers pushed it just a little too far: