[svachalek]

Not a biologist, but it looks like it's an urge toward certain behaviors that just get reinforced. A mouse tail running away from them is very exciting and they want to chase it. But a piece of string can trigger the same thing. They run towards it, they jump on it, they bite it, if they're hungry they might eat it and eventually the whole thing becomes a trained behavior in wild cats. A well fed domestic cat might get stuck on "let's be play friends forever" though.