There are many obedience trained dogs out there that are a little crazy when not in a show situation. My dog is also obedience trained, and works extremely well for me at shows. However, she knows the difference when we are at the park or just playing, and she behaves differently. You have to expect that.
I agree that you should have your dog on a leash when greeting strange dogs, you never know what can happen in these instances. After the dogs have had a chance to meet, then let them off leash. As for your dog just running off, well when they are off leash, that's going to happen. They are dogs. They respond to different stimuli that we cannot even detect. I wouldn't get mad at the dog... you either can live with it, or not. If this action bothers you so much, put the dog back on a leash and let him/her off leash in less doggy populated areas.

Best of luck! And remember to give your dog a big hug