Interesting question. Your team beat my time in a crucial game, my team is seen as being superior and therefore a target. Their team has an arrogance due to their excellent history and therefore need taking down a peg or two?
There are political reasons also. England’s perceived place in the British Union is an obvious (and understandable) one.
I suspect a lot of it is down to sheer tribalism. In the recent RWC, I was torn in a number of ways. England were very much underdogs and had an easy ride through the group stage. There was some entertaining rugby played, but there were far more interesting games for sure, and whilst I would have loved to see England play the final, I suspect it was a better game for their exclusion.
We are a big rugby family, with lots of us playing the game at various levels (including representing at national level). My family is Welsh on my mothers side, and Irish on my fathers. The immediate support for whoever is playing England by those family members both amuses and annoys me in variable amounts, and there is always lots of (mostly) good natured banter around 6 nations time, but in big internationals, those family members are always happiest when England lose and, to be fair, England have given them ample opportunity over recent years,
So take your pick. There are lots of reasons, but competition amongst the fans is as big as that within the team, and as long as it never turns “footbally”, that’s a good thing in my book.