Also, if you tank is a Shadow make sure they aren't using Noxxic's advice and using their combat stealth as a CD.
For the record, Noxxic's advice is *terrible* for shadow tanks. Really, really terrible. Using combat stealth as a CD is basically the only good thing in there, but it doesn't really help on this fight except to fix mistakes on Incinerate Armor (and then only on Hard Mode).
Coming back to the OPů There is no agro drop on any mode of the hover tanks. Here's a log from a recent one-shot we had on Nightmare Mode: http://www.torparse.com/a/58702/12
If you look at the Threat section, you can see a lot of upward spikes during tank swaps, and never a single downward spike (which is the hallmark of an agro drop). For reference, here's an older HM Kephess log using a strategy where the tanks trade off on the bleed (meaning both tanks get the agro drop at Kephess's jump): http://www.torparse.com/a/19420/11
One of the previous replies got it right: you're losing agro because of DoTs. This is super-easy to do if you don't tank swap on the IA, even when you're maximizing your threat. My advice would be to do the tank swap (it's really not that hard) and ensure that you're leading with your "high-threat burnout" rotation in the time before the first IA. Don't be afraid to AoE taunt (if you start on Stormcaller) to taunt boost and ensure that you really have a good hold on the boss going into the kiting phase.
On nightmare mode, if either tank loses agro on either boss for even a fraction of a second, it's probably an auto-wipe. Practice makes perfect!