Could be any number of reasons. Number one and two being he doesn't like either the physical parameters in the cage or more likely, he wants more space and room to explore. I don't like the sound of his feet on that harsh screen and if he is really persistent, he could pull out his claws and/or damage his feet. Can you place the cage up high and orient it horizontally, to provide more room for his to crawl back and forth? Chameleons like height, but they do more traveling laterally, than up and down. They enjoy looking down on their territory, so if the cage is high up, i.e. Above your eye level, then lateral space is best, IMO.