When a road surface is something other than smooth (i.e. rutted or grooved), the grooves in the tread "want" to match up and follow the irregularities in the road.

a. Bias ply tires do this more than radials, due to their construction.

b. Worn suspension does this because there is more slack before your steering input has any effect.

c. An alignment with negative [or less positive] caster does this because it lacks the inherent stability that positive caster provides. See also: worn suspension that can bring with it bad caster. See also: cars with a significant tail-up stance that bring one more degree of negative caster for every additional degree of rake in the stance.

d. An alignment with significant toe-in OR significant toe-out does this because you can't achieve "straight ahead" with both tires at the same time - one might be straight, but the other is trying to head off in another direction. See also: worn suspension that can bring with it bad toe settings, like a floppy idler arm does.