its a defense mechanism..if a cham gets scared, they cant really defend themselves from large predators, so they jump/drop. also, if they have nowhere else to go, and they really need to go somewhere, they will do it. chams are usually reluctant to do it at first, but once they realize it gets them somewhere, or out of danger, they will do it more often. and their bodies are actually made for it, i read somewhere on here that when they do that, they inflate themselves with air, and when they hit, they have a softer landing, and go on with their lives