I know the post is kinda old but I wanted to add my 2 cents worth.

I think that's a reaction to feeling vulnerable. For whatever reason, when you set scar down, he didn't feel safe. When you picked him up, he did feel safe. No mystery there. You also have to remember he probably couldn't see you all that well. I get the impression that many garters really don't see very well, but they do see motion, and of course, if you're feeling vulnerable, and you're a snake, you'll take defensive maneuvers against anything that moves. About the tongue going crazy.. unfamilar surroundings, smells, etc. can incite that feeling of being vulnerable, and so the tongue goes nutso looking for anything familiar and safe.

Could be that his habit of acting the way he did, instead of fleeing, is what earned him those scars! I can only hope that this spring I can end up with a garter pal with such spirit!