When a cat is not napping, there are only a few other things he is liable to do. He might eat some food. He might meow loudly for no reason. He might even lick his own foot!
But if he does not feel like doing any of those things at the moment, there is a good chance he will find another cat and pick a fight. That is what happened here, when Jack decided to invade Dave's personal space whilst Dave napped. Dave wasn't working on any new napping strategies at the time, so he didn't much mind getting into a fight.