Don't feel bad! It happens!
Who knows what started the last fight. One just could have gotten slightly tense and the other launched.
What's important going forward is you are proactive. Find a place to play fetch where there are no other dogs. He may have thought the other dog was going after his ball? And find a good trainer. I would look for a Schutzhund club so I found a trainer that was familiar with the breed or find a good trainer that knows about aggression. At 2 years old, he is maturing and that puppy attitude may be gone.
~The strength of one's opinion should not exceed their knowledge on the matter~