Out Of Character Aggression - Any Advice?

I own a 3 year old Border Collie, who is as sweet as they come. He's very well socialized, and we frequent our local dog park. I have a friend who owns two dauschands, and a lab that I often bring my Collie to visit with. This has been a weekly thing for almost 2 years. They have watched my Collie for me when I've been out of town, and we've spent the night there on many occasions. My Collie has always gotten along with her dogs, both big and little, as well as her infant daughter. I've never had problems with aggression, besides the occasional growl during intense rough housing. He's not a barker either. He would go for small animals if he got the chance, but that's typical of a dog.

However, this evening, while at my friend's house, my Collie suddenly leapt at one of the dauschands, and went for their throat. It seemed to be a completely unprovoked attack, and it has never happened before. He bit hard enough to draw blood on the smaller dog. I shudder to think what would have happened if we hadn't been there to pull apart the scuffle. Any advice as to what could have caused this sudden act of aggression? Should I be concerned that this will be a repeat incident? Thank you in advance!