One thing I did read was that many of the incel types have had experiences at highschool being bullied by girls. Also if you look into it, girls tend to bully in groups, while boys tend to bully one-on-one.
https://middleearthnj.wordpress.com/2013/05/06/gender-differences-in-bullying/Girls bully in groups or packs by using emotional violence. Girl bullies often use their popularity to encourage other teens not to be friends with the victim. A popular girl bully will expect and require that the other girls in her group back her up
So, if you have this kind of experience where one or more 'alpha' girls is bullying you as an unpopular male in class, and it definitely happens, then you can imagine the kind of emotional scars this could lead to. It can lead to ostracism within the classroom, and males are subsceptible to the negative effects the same as girls are, and they often lack the same sort of sympathetic support networks that girls can tap into.
There is also the fact that if a guy talks about being bullied by girls he's probably going to be mocked for talking about it in most places: even in feminist circles where they tout how they 'laugh at male tears'. So, you have these people with embittering experiences, they have no place they're really allowed to express how they feel, then they discover the 'incelverse' where people are sympathetic to their experiences.
This is the real broader issue. We talk the talk about allowing boys to be vulnerable and talking about feelings, but when it actually happens and they start blabbing about how they feel we feel revulsion and herd them into a corner, then wonder why it becomes toxic.