In the wild, cats are known for toying with their prey before killing it. Domesticated cats have the same instincts, so any small object the cat paws at might decide to “run away” and turn into a fun hunt. Cats are also very curious creatures – they learn a lot about objects by feeling them with their paws. If feeling an item leads to it falling, then they get to watch how it falls, which is interesting for them.
Cats also tend to learn very quickly that knocking things over gets the attention of their human. Just like an child acting out in order to get attention when they’re feeling ignored, cats will knock things over to get their human’s attention when they’re feeling bored or restless. In this case, if you ignore your cat every time he knocks something over, he may soon grow bored of the game when he sees it isn’t getting him the attention he’s seeking.