We had an Arizona meet-up last year, and Joey did the same thing, first time I ever saw him do it too. There was a big bubble on his mouth at one time. I took him to another meet-up with lots of dogs a month ago, and the same thing happened, but not to the extreme extent as the first time. Joey's breeder thought it was an anxiety issue and just presenting the situation more often would make it better each time. She did say that when this happens don't coddle, and baby him, which is exactly what I did, this makes the dog think that there is reason for him to be scared. Joey doesn't do this normally so, I haven't had reason to treat it, and I believe if an underlying medical issue were involved it would happen at other times. If this only happens when you leave the house, I believe separation anxiety is the culprit, and there are behavior modification steps to increase his confidence. Once of our member's by the name of scootiebootie, offers step by step advice for those people who need extensive help. You might want to contact her.
http://www.yorkietalk.com/forums/mem...tiebootie.html
He's a little doll, by the way!