It's pretty easy as a MCH to steal hate from a tank if they are pulling singly and not following up with an aoe or 2 to solidify their status on the rest of the connected mobs. If I notice the tank isn't actually hitting all the mobs I try to stick to the ones he did hit until he starts aoe rotation.
edit: It's really an issue when the tank is doing a running pull by only using shield lob or provoke and you toss an aoe at the group and half of them turn around and stop following the tank.
edtit 2: The same thing happens to healers when they try to heal the abovementioned single shot run and gun tank. I don't know how many times I've had to turn around to pull a mob off the healer who was just trying to keep the tank up through his pull.