Cooper started doing this too, after over a year of riding in the car normally.
I always feel like it is because he is unsure of what is going on and he is scared. We just ignore him, because the more we coddle him the worse he acts. He just is not comfortable in the car at all, but he doesn't want to be left behind either
The main reason I think it is just nerves is because, alot of the time after we have had our outing and he understands we are going home, he doesn't do this.