Fearful, Protective, or Territorial?

[HarleyTheGSD]

Varick isn't very social with people. He isn't scared, he isn't aggressive (though he is very wary) he just doesn't want to be petted by people. He does fantastic in large crowds with lots of noise and movement. Yesterday, after I put him away in his crate, (in the truck) a man walked up behind me abruptly and Varick barked a very loud, growl- like bark. I told him "NO" and he stopped right away. I'm not really concerned, this to me, seems like he might have simply been spooked. But I'm thinking it could be territorial or protective behavior. What do you think?