Question about Aggression

My 14 month old was playing in my backyard with 2 other neighborhood dogs when Chivas just kind of went off on one of the other dogs. I didn't see what happened, if the other dog did something or if he went for one of Chivas' balls. They have played together lots of times before this and have never had a problem. This was the first time that we were in one or our back yards. Usually they just play in the street and front yards. He also goes to the dog park at least twice a week and never has any issues. It scared me a bit cause I have never seem him act like this and really don't like it.
He isn't fixed yet...could that have played a role? Should I be worried?