Brexit probably hasn't helped. It feels like that, combined with Trumps win in America, made racists feel empowered that they could be more open about their beliefs because there were two big right wing victories almost back to back
This doesn't mean that if you voted for Brexit then you're a racist of course, because that's a ludicrous notion that only does more harm than good, but most racists probably did vote for Brexit and will have seen the Brexit victory as a victory for their beliefs and life style. Hate crime numbers went up following Brexit for instance, which suggests racist people felt like they had more of a license to be racist in public
There'll be other reasons as well though. For instance, footballers being so wealthy and seemingly so detached from the life of the average supporter has led to a much frostier relationship between footballer and fan, which makes it much more common for supporters to resent players and indulge in poor behaviour as a result of that resentment