Why does my dog seem to growl at this one dog?

My lab cross (female neutered) is one of the most friendliest dogs, loves to run up to and play with other dogs, never any problem with her, however she seems to have a nemesis with another dog we see from time to time, I think the dog growled at her once and from that there has been mutual growling. Today after our walk I opened the boot (hatchback) to let her out and she got out turned and bolted down the street, I'm not sure did she go up to sniff the dog it happened so quick but the two were fighting, I do not know which dog instigated the fight. As the other dog was on a lead and the owner was pulling it back my dog seemed to be the aggressor. I have seen my dog attacked by other dogs and she always looks really surprised and never fights back, with this particular dog I'm not sure. I got a telling off from the owner for letting the dog out without a lead even though I explained I had just taken her out of the car outside my house, (where 99.9% of the time she goes straight to the door and when she is in her cage in the boot I don't like having her on a lead in the car. I always walk my dog on a lead in a public place) when the other owner said if your dog is like this she should be muzzled, I did apologise but why this one dog?