If they're doing it less often than every 2-3 minutes, in certain circumstances where there's no way you could open up enmity as fast as they could... it's probably because they're being foolish and/or think about how to assist mob movement (or 'tanking').

When it's consistent... because they're impatient and/or QS/EJ has too long a cooldown, or perhaps you're opening with Goring, Storm, or Soul/Del when you have too few oGCDs or too little attack power to actually hold threat during that method of opening.

(I often early pull as a dps. I just don't get hit more than once in the process, and by the time the tank reaches the mobs, s/he has threat on them.)

As DRG I often single target a key mob while the BLM/SMN AoEs. This usually isn't necessary any more often that EJ, so all good with a stored Dive to get back. When I'm the BLM/SMN, QS. Aside from it being wasteful for both to ST or their being too much urgency for both to AoE, threat reducers shouldn't be needed. If you're expecting them to pop QS for anything more than their popping every CD they have while you don't have any offensive CDs of your own and/or their attacking in a different manner than the other dps, pushing both ST and AoE extremes at once, you might just be doing too little enmity.