I was playing with my dog off leash and when I released him to get the tug he ran past me and attacked a dog that was walking past our house. Did playing tug activate his aggression?

From playing tug to attacking another dog: I have 1.8 border collie who hardly ever shows any aggression towards other dogs. I've been working on playing tug with him outside, with distractions, and this morning we had a great start to the session, with him running after me from sit-stay and grabbing the tug with energy. But then a dog passed on the road near our yard (whose door was open to let in more distractions) and when I told him to 'get it' he ran past me and to the dog (who by this time disappeared around the corner) and attacked him - all show and no biting - but scary and unacceptable. I know the fault is mine for not putting him on a leash. That was stupid. My question is whether in your experience the tug play activated some drive to go after the dog with such aggression. In other words, is it the circumstances or is my dog likely to be changing as he is maturing, and if so, what preventive measures can prevent development of aggression (besides not having opportunity to repeat)?

Thank you very much. This helps!