I'll give this a try. I've been using @pet on my hunters MD macro since I figured directing all of the threat to a single pet would be a bad idea if it dies and the pets that are still alive don't have enough threat to pick up the boss. You don't have an issue with the boss switching to you after your main tank pet dies (which I assume it does often if you're MDing all agro to it)?
So I just tried MDing to one pet. It helped the boss stay on that pet until it died and then it switched to one of my druids rather than another pet. Maybe the problem is balance druid is somehow doing more threat-per-second than the hunters even though it's doing half the DPS. Weird.