The reasons for this is that other realms only need to switch over to commit player versus player actions but also have the ability to simply teleport back to their own realms when they begin to face a serious resistance.
Take the entire realm for example: Alliance mainly from Sargeras are crossing over to ours simply to camp weak and leveling players. And of course when backup arrives they immediately flee into the air or even back to their own server.
I know this is affecting other realms; mostly world PVP servers the most although RP realms are affected as well because of people from other servers intruding on their role-plyaing and ruining it in general.
I propose that cross realm zoning should be removed from World PVP and RP realms as others simply ruin the communities fun.