Enemies attack you if you take an action other than fight, evade, parlay, or resign. They may have played an asset with the "Fast" keyword like Magnifying Glass or Switchblade, in which case no action is required.
But also, some assets have abilities that have action designators that mean activating those abilities doesn't trigger AoOs.
Plus, some assets have abilities that are free triggered abilities, these also do not trigger AoOs.
And, some assets have passive abilities that are just always active.