I've never been very impressed by him, but I get the sense that since Jose made his remark about having a small squad we started looking for obvious players to be cast off. Blind, Mata, Herrera, Valencia, Darmian, Fellaini, etc. all have had excellent moments for us and arguments can be made for or against keeping them. With Rojo, at best he's been solid if unspectacular, and the consensus is he isn't good enough to start at CB or LB. Since he'd be a backup and would be blocking young players people are attached to like CBJ and Tuanzebe from playing during an injury crisis (plus the general skittishness about Jose abandoning our young players) he's an obvious, uncontroversial candidate to sell. The best argument for him is "he can play multiple positions," but he plays the same spots as Daley Blind without the passing range. My guess as to the meanness is that people have realized that almost everyone agrees with the idea of selling him, so there's no immediate backlash for calling him out. If you are vocally aggressive about Mata or Rooney, you'll split opinion and risk being donvoted. With Rojo, you're pretty safe from that because no one is going to rush to defend him.