Wow that's weird. Dogs sense things that we can't. He probably smelled something, possibly a wild animal. Or, maybe there was a wild animal that is dead nearby. Try walking him again and see if he does the same thing. But, take someone with you just in case he sensed something dangerous.
It's funny how dogs sense things. One time we had a Blue Tick/Lab who was always friendly and loved everyone. I never saw her not like anyone, she was just a big loveable bear. But, a cable guy came to fix a problem we were having, and she acted like she wanted to kill him. She would even stand in front of me and bark at him, like she was protecting me. I had to take her to another room and calm her down, but she really did NOT like the guy for some reason. I'm sure she had her reasons, but we couldn't figure it out.
