If someone sees you transform back into your human form, then you will get a bounty, because people now know you're a werewolf. This was implemented before Dawnguard.
Well if any NPC sees you going into or out of beast form you'll get the bounty. So i would also assume that's why he kept attacking you.
I know that, I have been seen transform back before and had people attack me, but I have never had them yell at me calling me a beast. I thought maybe that was part of the changes they made to werewolves in Dawnguard.