Quick answer: Cats knock things off tables to get your attention, to test objects using their prey drive, or because the reaction you give reinforces the behavior — it's deliberate and learned.
If knocking something off brings you running or causes any reaction, your cat has learned this behavior works. Cats are motivated by any attention response, including negative ones. The most effective response is to completely ignore the behavior and only engage when your cat is calm and not demanding.
Cats paw at objects to test if they might be prey — a falling object triggers their prey instinct. This is especially common in under-stimulated indoor cats that don't have enough hunting outlet. Two 10-15 minute daily play sessions with a wand toy address the underlying need.
Ignore the behavior completely — no verbal response, no eye contact, no reaction. Provide acceptable pawing alternatives: puzzle toys, toy mice, crinkle balls. Remove objects you don't want knocked over. Consistency in ignoring the behavior is what changes it.
Ignoring the behavior and increasing enrichment resolves most cases. Your reaction — even a negative one — is what sustains this behavior, so removing the reaction removes the motivation.