Some people argue its because of people who like to gank the back of a siege line or troll, however there's more layers to the xv1ing than just that. Some large groups will just chase people they see on sight, they might not even be doing anything or anywhere near an enemy objective but groups will just chase after them which I find weird. I was once in a stream of someone playing on EP ( I was on DC) and he wanted to 1v1 me and so I went for it, however there were people trying to hit him with meatbags or snipe him from behind even though we werent near any objective, it was just an open field fight, yet people wanted to go after him. I think it's fair game to x anyone who wants to stop you at an objective but I think it's toxic if they're not near an objective or really doing anything at all and you just chase them down
I hate when players walk by 1v1s in cyrodil. Look, the faster you help me kill this guy the faster we can move on to killing other players. Nothing more annoying in Cyrodil than being stuck in a stalemate fight because fellow alliance players run past out of some dumb notion of respecting a 1v1.