To clarify a little beyond kanras' less than verbose explanation, this is most likely because there's meant to be a limit to how much threat you can generate with the first thing you hit a mob with if nobody else has aggroed it before you. There's no bonus for being the first to tag something. This didn't work, and now it presumably does. Basically, whatever a mob gets hit with first can only generate, say, 100 threat (example). If you pull it with Drowsy which might generate 500 (example), then it'll only generate 100 if nobody had aggroed the mob before you. This was exactly to prevent someone from just pulling with some huge-aggro spell and having the mob stick on them long enough for everyone to kill it.
The reason for the widespread belief that initial aggro has some kind of bonus is that you actually need more threat than the current tank in order to take aggro from them. A mob won't turn from its target until someone else has generated some amount in excess of whatever its target had. If the tank currently has 1000 threat on the mob, for instance, it might take something like 1200 for someone to pull it away.
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