dog baring teeth at people

Question
I've had my 1 1/2 year old German Shepherd/border collie/lab mix boy since he was 9 weeks old.  He seemed somewhat fearful of new situations as a puppy, but overcame them.  He was always more interested and attached to other dogs than to people.  He has always had a habit of trying to lick every dog's face. Over time, his play with other dogs at the dog park (an open looping 3 mile trail) has become more aggressive. He persists when the other dog has had enough and ignores my commands to "knock it off" and won't come when I call him for time out.  He also barks more often at certain people on the street as we walk on leash, even neighbors he knows.  He's fine with other people.  Several days ago, he started getting out of my yard.  My next door neighbor called me saying that my dog had just bared his teeth at her husband.  I then found my dog on her property, with a police officer trying to snare him with a pole and loop.  My dog came to me when I called him and I put him in the house.  The officer told me that a woman walking her dog reported that my dog bared his teeth to her.  I did not witness either incident.  I was stunned by this, as my dog has NEVER shown this kind of aggression to a person.  I don't know the precipitants.  My dog is big and wiry, 80 lbs.  He's never been abused as far as I know.  I am affectionate with him, exercise him daily at the dog park, often with his dog friends.  He's usually been calm, is well bonded with me, has never been destructive to property.  I obviously need to reinforce my fencing ASAP and I've called a positive reinforcement dog trainer.  Is there any light that you can shed on what is going on with my dog.  On the morning that this occurred, he wanted to be let out early, and because of insomnia, I fell back asleep.  He may have been out in my yard for an hour to hour and a half, much longer than usual.  

Answer
If he is not neutered getting him done might help calm him down. Now he is around the age where will 'test' everyone,to see who's in charge,and it sounds like he might be more on the dominant side. I am glad you are getting him to training,as I would say that is the best and right thing to do right now,as this could become a dangerous problem if there are small children around,as growling might turn to biting. And the training will also show you how to correct this problem before it gets out of hand