That is thermoregulation and you have already described the two things that make it so. It happens while basking, and he shuts it when he moves to the cool end.
Uros want a basking spot hotter than most pet lizards get, and once they are up to temperature they vent a bit of heat through an open mouth. The version that worries me is a gaping lizard everywhere in the enclosure, including the cool side, because that means there is no cool side and the whole box is too hot.