What causes a toy car to turn when you push it across a flat surface?

I was just playing with a toy car of mine. The wheels don’t turn on the car, but whenever I pushed it across the kitchen counter, it also turned to the left or the right. Why doesn’t it ever go straight?
Image by Sappymoosetree
Tags: flat, across, causes, Push, surface, turnRelated posts:
the parabola of your arm pushing the car, and the uneven friction of the wheels
One of the wheels are not loubed as good as the other
One of the wheels is not turning as fast as its counterpart on the same axle.