Dogs don't have to roll over to be submissive. Just laying down, not 'crouched to leap" can be very submissive.
However, if this is the ONLY dog he reacts to, maybe he picks up something you aren't with the dog. Maybe the dog lives with other animals that set him off. If you can, find out from the other if OTHER dogs respond to it that way and what other types of creatures live in their home. I nearly caused Arwen and Cara to have a stroke one day when I came home and had been holding a lamb.