Its been a couple expansions since I tanked, and I might dust my DH off this expac(healing atm), but it has to be a 2 part issue. Holding aggro is so easy nowadays that I find it hard to believe people can reliably pull aggro consistently.
The only thing I can guess is your attacking a mob before the tank, out of his AoE, and hes not seeing it to taunt it off you fast enough. Meaning, your both at fault. He should be able to just taunt the mob off you when it starts to run after you(but lots of people have a terrible tendency to RUN from the mob and the group instead of to the tank, making chasing it down and taunting it a chore). And, you should at least hit who the tank and the group is targetting instead of just randomly picking a target in a pile of mobs. Even then, with how easy it is to hold aggro, the only thing I can guess is your snatching this single monster way before the tank can hit it and its just avoiding the group.
But your melee, so I dunno. Again, Ive healed both of the last expansions and tanked in BFA. But when I tanked, what would happen is some guy would randomly hit some straggler mob and pull aggro before I could catch it in my aoe with the rest, mob runs after them, and I taunt it off them. If its more than 1, it can be a problem. And, some tanks may just not be very good and just not see it because they are focusing on something else, so you get plowed, and then run away from the group and make it even harder to get the mob/s off you.