Apologies if this has been done already, I searched and found nothing.

While doing the vampire questline; just after the "Bloodline" quest, I was being led by Garan to explain the bloodstone chalice when all the vampires in the hall suddenly started attacking without reason and killed me. I tried re-loading a save, but to no avail.

Eventually, I toggled God mode, ran out of the castle and travelled to Markarth where I had a (small) bounty. After paying it off and returning, everything continued as normal.

I'm guessing this might have something to do with how the characters are set.

Sounds like the caught by friendly fire fights which followers get into so often.
I wonder what set this one off.