I can think of a few possible reasons:

 1. Starvation or dehydration
 2. Committed suicide (self-inflicted)
 3. Died from wounds not inflicted by zombies (non-self-inflicted)

I want to emphasize that if they were bitten by zombies, they would eventually turn into zombies themselves and wouldn't be found just sitting in their cars dead. For the TV series, I'm not sure if any death would lead to them turning into zombies. But it doesn't look like it would, since the dead are in their car.