I've had this happen to me, it a conjuration bug of some sort. All of those creatures should have been raised by you at some point.
^^
this is what's happening. What ever you raised while on your travels will spawn in with you sometimes after fast traveling, the most I had at one time spawn in with me was 15 NPC's. The Xbox wasn't very happy but it was epic all the same, watching all the towns folk running around, all the guards taking up arms, Whiterun will never be the same again
