There are many situations like this. First, he may suddenly remember something he has forgotten, so he turns around and goes back upstairs. Second, if there is something between you, he may not want to Meeting you, third, he has a crush on you, but he is shy and gets nervous when he sees you, so he runs back again, so these are all possible. If you want to know, if you know him, you might as well ask him directly. Guess There's no point in guessing.
Hee hee