The car has a bad tendency to follow the contours in the surface of the road. Depending on the grading of the road, the car will tramline to the left or to the right. If the highway is perfectly level, the car will drive straight as an arrow with no pulling whatsoever. (The tires and the psi are normal).

 

Have the alignment checked, even though it doesn't pull when the road is flat. Could be caster is too high, but it's equally too-high on both front wheels. Or some other angle, that's wrong but equal side-to-side.

Did this happen with new tires? Some tires are more prone to this than others. Occasionally you can improve it by adjusting tire pressure - maybe a bit higher in front & a bit lower in back.