I’m guessing biological warfare.
Cars takes place on a world where human technology had advanced past our own and had invented artificial intelligence. They had giant factories that could build sentient vehicles like the ones we saw.
Then the terrorists struck and released Agent X which killed every carbon-based lifeform on the planet. But the robots were unharmed. And the factories (which are also artificially intelligent) are still building new cars which are now programmed without the memory of mankind.
But what are the cars going to do? They lack true creativity so they cannot form their own society. So they just go through the motions of duplicating the human world which is gone. Hence the theme of longing for a lost past that permeates the movie - the cars are all waiting for the passengers that will never return.