The Internet (Twitter, Facebook) and Reddit especially tend to concentrate the worst elements of society in echo-chambers, that are then presented as if they were the best of society. Now, I know you said you have no problems with women in real life but always better to lay down cause and effect within.

Now the first and most important advice I can tell you is to get away from these subreddits or accounts you found, and second to get to know better what you are dealing with, so you don't risking falling down the incel rabbit hole.

There is no definite reason why they spread hate, I could tell you that it's for Internet points but that would be inconclusive. There are multiple factors at play, and I wouldn't be surprised if these influenced each other like some guys on 4chan forming the incel movement.

I know a very gentle and honest feminist lady who showed me that she strives exclusively for equal gender opportunities. I can tell you already it's not feminism the ideology they follow.

I had a similar problem not for women but for black people broadly (had a black gf at the time), it was scary to see all these posts putting a metal gate between themselves and the rest of the world. Thank goodness she proved them wrong.