To unlock 'Why So Hostile' you must destroy 200 civilians cars.
If you want, this is easily possible within the first few levels of story, just hold down B to fire your machine guns whenever your not attacking enemies.
If you didn't get this during your play through as you were aiming for scores, then load up 1-1 with your upgraded Iris. You can basically hold B on this level, and there are a couple of hundred cars to shoot here. If iris has Max machine guns and missiles, you'll get this in a few minutes on 1-1.
Also, using the truck on level 4-2 counts and is an easy way just by ramming oncoming traffic.