She's a 10 year old rescue shih tsu. When I carry her by her armpits with her facing forward (the direction I'm walking) she opens her mouth. When we carry her with her facing behind me, she doesn't open her mouth. She also doesn't open her mouth when she's walking normally or anything. She can jump on her elevated bed fine (1 foot off the floor).
My dog does the same thing. He is also a rescue and lived the first 7 months of his life fending for himself in the middle of nowhere. I am wondering if it is their version of “playing dead” as I assume he had probably been approached by wild animals many times over that seven months. It almost seems like a defense mechanism because he will just go to dead weight and crack his mouth open whenever I have to lift him!