I think some of it may be that thing where so many fans think it's not good enough for their favorite character to simply be their favorite, but they need to be shown as objectively the best somehow. A lot of people (myself included) like warrior woman characters and watching a woman kick some ass. For some people, dressing it up in pseudo-feminist claims provides a way to 'justify' their preference, as if it needs justification. (Actual feminism can be in favor of having female characters who are capable in a fight, but not deciding that every single female character who doesn't demonstrate impressive fighting skills judged a failure.)

Some of it, of course, is people happily jumping on anything that gives them a handy excuse to hate on female characters.