I would imagine the simplest/most common reason is that they are browsing different rooms, trying to decide which model to visit. The thumbnail images are helpful, but you really have to enter a model’s room to get a clearer idea of whether it’s for you or not.
Having said that, I wouldn’t take a quick departure too personally. Sometimes guys are just in browsing mode, and aren’t going to stay anywhere for more than a few seconds even if they like the model.
When I browse, I go through multiple rooms and make a quick decision whether to follow the model or not (if I can’t decide quickly, I err on the side of following them so I can return and spend more time later). I’ll follow a bunch of new-to-me models at the same time without staying in the room too long, with the idea of paying each of those models a proper visit once I am no longer browsing.
