Please help out a confused old person. Why was 'Social Justice Tumblr' given a 15th seed (one below 'Facebook with Your Republican Relatives')?

I thought social justice was a good thing. I know the term 'Social Justice Warrior' is used as an insult by MRAs and other supporters of bigotry... but Jezebel staff wouldn't be part of that crowd. I am admittedly not very familiar with the 'Tumblr' aspect of it all.

Why is 'Social Justice Tumblr' a bad thing?