Just a heads up but +15 enmity is worthless (especially considering some skills generate up to 2k enmity)
What exactly were you doing that would generate enmity? What actions etc.

Some mobs also have hate reset moves if you didn't know (one of them in the final BRD quest does as far as I remember)
Also that's how mog fight is supposed to work. Some will stop attacking someone to go attack someone else. This is why you need to bind/kite some. It isn't because you generated massive enmity but because that's how the fight works.

I'm not sure on your problem in cass hollow though. Unless if you go into detail into what skills were being used I can't determine whether you're just generating more enmity than the tank or if it's genuinely bugged. Right now it looks more like bad hate management but that's only because I've never ran across this problem (apart from where it's programmed into the fight like mog, buffalo, several final job quests etc)