Why this German Shepard?

I have a three year old very obedient German Shepard. He never strays and I totally trust to be left off the leash while playing outside. This morning we were playing in a field by ourselves, and all of a sudden he took off running! He was out of my sight so quick. He has never left me, he is always right by my side. It took about 30 min for my kids and I to find him. He was just strolling by the lake by himself without a care in the world. I was so scared and upset. Why do you think he left me? Im constantly tripping over this dog in my home cause he follows me everywhere. Im confused