Why do cats knock things off tables?

There is no single cause, and several plausible ones operate together depending on the cat and the moment.

Investigation is the most likely baseline. Cats explore objects with their paws, which are highly sensitive, and pushing something is a way of learning what it does. Wild hunting behaviour includes pawing at potential prey to test whether it moves, and a stationary object on a table invites the same test. Something that then falls, rolls or makes a noise has behaved in an interesting way.

Attention seeking is the reinforcement layer, and it is usually what turns an occasional behaviour into a persistent one. Knocking a glass off a counter reliably produces an immediate, animated human reaction. From the cat's perspective that is a successful outcome, and negative attention functions as attention. This is why the behaviour often concentrates around times when the cat wants feeding or company.

Boredom contributes. A cat with insufficient stimulation will find its own entertainment, and a shelf of movable objects is available entertainment.

The practical responses follow from that. Reacting less, and specifically not reacting immediately, weakens the attention reward, though this is easier said than done with breakable objects. Removing valuables from reach handles the consequence. And increasing play, particularly interactive sessions with a wand toy that let the cat complete a hunting sequence, addresses the underlying need rather than the symptom.

Punishment is ineffective and tends to damage the relationship without reducing the behaviour, since it supplies exactly the attention that maintains it.