I think you see that with the idea that a lot of people on the left are just white males and not much else (even though this is false and it is very diverse). The powers that be will try to paint a movement, or what have you, that goes against their interests as being dominated by privleged people and/or people from a historically dominant group or having some type of bigotry and/or reactionary views, especially if there basically nothing else to legitimately criticize them on.

As you mentioned, there was the idea that Bernie Sanders, as ran to be President, was some sort of sexist, even though his record and stated positions do not support. A lot of the people who said it either knew they were making a proverbial mountain out of a molehill or are the type of people who think the slightest opposition to a woman (such as competing against one in an election or just disagreeing with) is sexism in and of itself.