As anyone with a cat knows, cats are curious creatures (or just plain obnoxious) so whenever they see anything on a table or desk, they can’t resist the urge to knock it off. While some cats may do this for malicious reasons, most of the time cats are just trying to have fun.
If having your cat knock things on the floor isn’t your idea of fun, you need to find a way o modify your cat’s behavior (good luck with that). First, try to give your cat other activities to pursue rather than exploring your desk. Second, clean off your desk. Third, get a dog.