Well from a gearing/enchant/gems, your setup is pretty solid. Your exp is over soft cap and your hit is very respectable. Without knowing more it's hard to speculate, but if your OT is a pally and we are talking about multi-mob tanking then the scenario you describe above is somewhat common place.
Warriors are front loaded for threat pretty well, but compared to a paladin for example, over the course of a pull it's hard to out AoE threat them. There could also be alot of other reasons including rage issues, mis-taunting, or improper rotation usage.
Make sure your using SS & Revenge as often as you can as these will really help you build threat and keep aggro. On single targets, you should have no issue holding a boss for example provided both your OT & group realize you require a few seconds to grab aggro.