As Lotl referenced, the cause is conicity - root word *cone*. Conicity is a tire force pushing (pulling?) sideways. All tires have some value of conicity (although that value could be zero)
Every vehicle has a threshold above which the vector difference of the front tire conicities will cause a pull - and below which it will not.
For example, if a car's threshold is 7 pounds, I could have a pair of tires both with +10# conicity and there will be no pull, but if the tires are +4 and -4, there will be a pull. As you can see, it isn't a matter of a "defective" tire - it's the difference in conicities.
HOWEVER, a vehicle's alignment can cause exactly the same symptoms. A simple test of swapping the front tires will determine which it is:
1) If the pull doesn't change, the problem is 100% alignment.
2) If the pull completely changes direction, it's 100% tires (conicity)
3) If the pull disappears or changes (other than completely changing direction) - as is the case here - the problem is both the alignment and the tire conicity.
Please note, that tires can change conicity if worn irregularly.