I can sympathize with this, even if just from more of any annoyance stand point. Dps can go AoE crazy or focus down targets all they want when I'm stopped and running my own rotation, but it's a different story if I haven't finished the pull yet.
It can happen when a tank is approaching a pull with 2 or more sets. 9 times out of 10, this usually goes just fine, because most Dps are considerate enough to wait for the Tank to actually complete their pull before they start the heavy metal music. Unfortunately, sometimes you run into this one Dps who insists on going balls to the walls on the first target you touched before everything is in position. Instead of a nice, clean, efficient, and fast pull, you end up with the tank running to the next set, the mob chasing the tank, and "that dps" chasing the mob while they desperately try to upkeep their dps (which they typically fail at). If they pull hate, then the Tank has to turn the whole train around just to go backwards to save the squishy idiot who couldn't wait 2 seconds for you to settle everything into a nice orderly pile.
The absolute worst cases of this are when the Dps in question stuns the mob you touched. You end up getting to the end of the pull, got everyone nice and lined up, and you're about to pop CoS .... and then you see that one mob way back in the distance who's still trying to catch up with the group cuz someone stunned him.
Why do they do this? I dunno. It doesn't increase dps (it actually lowers it since it's unlikely they'll score positoinals or AoE's on a moving set of mobs), it doesn't work for the other dps (since mobs that are out of place get missed for AoE's), it hurt's the healers Mp pool (since a longer, less efficient, pull = more healing required), and reduces Tank enmity and mitigation (for the same reason that the healer's mp pool suffers).
Thankfully, this is a rare flower in the garden of Dps players. I get players who apply DoT's and get the odd hit off on in-progress pulls all the time, but that special snowflake who thinks that it's a good idea to go throttle to the max before the tank has stopped moving is not as common. It's just a little irritating when you see that kind of short-sighted behavior, and not just to the tank (I've seen a smn bitch at a Mnk who did this, because their AoE's were consistently missing the targets he was pulling out of the group).