Hello, yes he is on his lead when this happens, usually it begins with both dogs eager to sniff each other, then it seems to explode in a split second. I understand that makes it my fault in a way but with it only happening once in every 50-100 dogs its easy to assume he will be ok.
On this occasion the other dog was off his lead and his ball was rolling down a hill towards our dog. The lab came into his space and it kicked off pretty quickly.
On other occasions though, specifically in puppy class, it happened weekly with the same dog, a small cockapoo that was always just sat minding his own business, albeit fairly close.