Part of it comes from the fact Sunderland and Everton have been relatively close in recent years (obviously all relative). With your gesture for Bradley Lowery etc. Obviously Pickford as well, and all the Everton players we've had as well. They can't stand the idea that they're not everyone's second team.
But generally Mags I know think you're "bitter" about their success and jealous (as are we apparently).
I think your fans online have also been especially outspoken about the moral bankruptcy of their supporters, which they can't live with.
Plus they're all fat, scruffy, horse punching bellends who are basically Kopites of the Northeast