Writhing Horror does not have a problem with too much much dps. Your problem is that you tanks do not produce enough threat. Like any boss anywhere if your tank can't keep up with the threat per second of a dps, the dps is going pull the boss. This boss simply has an explicit mechanical penalty for this. Combat logging can be used to parse threat per second, have your tanks work on their threat rotation just like you would have a dps work on the dps rotation. If that's still not enough, try replacing your tank's endurance color crystals with power. Try having your tank burn an attack adrenal during the boss opening. Make sure your highest threat party members are guarded. Make sure all the dps know to use threat drops and when.
No, it has to do with getting the little adds with a Jealous Mate at the same time as his <10% HP mechanic where adds just start showing up.