Was Samson in the house with him? Was there any altercation? (this would be my main concern for if/when he goes "off" unless Samson got out of his way...and his rage hopefully wouldn't be totally redirected)
That just strikes me as very odd...but I've never had a dog with any negative tendencies toward humans. Even Dan's dog-directed reactivity is calm...he'll do a sit/stay two feet away while I pet (just doesn't want the dog in his face).
I hope somebody has some advice for you.
Oh, wait....wait, wait, wait....
Is he presently on an anti-inflammatory? Check for side effects. I know Rimadyl can cause aggressive behavior.