Fell Cleave generates 500 enmity potency on its own, and 750 under berserk.

Using a combo'd Spinning Slash, you generate 770 enmity potency. A follow-up Power Slash generates 1650 enmity potency (these numbers are if you stay out of Grit; you can swap into Grit if you really need to consolidate your lead). On PLD, it's a similar story, except that SB is 700 enmity potency and RoH is 1300. Either way, you're generating significantly more enmity over two GCDs than the double FC under berserk (although I'm not really sure why you would time Berserk to be active just before you were due to hand over aggro in a tank swap).

It's actually not Fell Cleave that messes up tank swaps. It's Butcher's Block. What typically happens in these cases is you see the FC go off, but it then gets followed up by a 1540 enmity potency BB (for that sweet extra 20 potency over the SE combo). The swap failure then gets misattributed to that sick crit hit on the FC. It's not your WAR's incredible dps. It's the 5.5x enmity modifier tacked on their dps combo.

Tank swaps are a bit like passing the baton in a relay. There's a shared responsibility on both tanks to make sure that the transfer occurs smoothly. If your co-tank starts sprinting off with the baton instead of handing it off, that's a problem on their end.