Cats are nimble and said to always land on their feet, with extremely quick reflexes and the ability to squeeze into any space where they can fit their whiskers.
They're obviously not clumsy animals, so why are they always knocking stuff off our tables? Are they actually just being jerks, mocking their humans by breaking our stuff?
Scientifically speaking the answer is no, and according to research done by Katherine Houpt, a professor of veterinary behavior at Cornell University, cats knock over our stuff to get our attention.
They also like to swat stuff off tables because of their predatory instinct to strike with their paws, so the swatting could actually be seen as a kind of training exercise for hunting. And Pretty soon they'll be hunting for a new place to live if they keep knocking stuff over!
-Via Thrivewire
Bah! It's because they're jerks!