Dog went crazy at smell of a jacket

Question
My friend Steve lives with me and he has a six month old German Shepard named Capone.  Capone hasn't been trained for anything professional at all.  My mom's cousin left his leather jacket on a chair when he came over today and when Capone smelled it, he went crazy.  He started fiercly barking at the jacket and seemed to be very afraid of it.  My mom's cousin didn't do anything mean to Capone and he was only here for about 30 minutes.  We had him smell a bunch of other leather stuff, including other leather jackets, and he didn't have that reaction.  Why would he go so crazy over the smell of his jacket?  There was nothing in his pockets.

Answer
Maybe he detected a scent of another animal on the jacket, the scent of that man triggered some memory, or something else spooked him and it was coincidental.  It is really hard to say for sure especially not personally knowing the dog or his history.