Some addons can cause NPCs to act hostile while they shouldn't and appear friendly while they're attacking you.
I had this happen for example when I went to turn in the polar bear (Hyldnir Quest) daily, upon mousing over the quest NPC on turnin the little sword appeared and she refused to let me complete my quest.
Another similar situation is when I went to Skettis with my priest to see if I could get Terokk's trinket (yaknow the one that turns you into an arakoa?), I drank the shadow elixir but all of the time-lost arakoa showed up as friendly. They had no problems attacking me however.
Turning off addons through the addon control panel didn't work. What did resolve it for me is to log out and back in. The addon that seemed to be causing this issue for me was mountiful, but I've got the suspicion that there are more out there that cause this.