The most likely explanation is your car has a problem with the wheel alignment and should be taken in to be inspected and fixed.
However, you can also pay attention to some other factors that could explain it:
- Slanted roads. Does it only happen on certain roads? If so, the roads could be slanted in that direction.
- Tire conicity. The tread on your tires might be worn and have a defect called tire conicity, where they are not aligned properly with the belts under the tread of the tire.
- Tire separation. Your tires might have had air force the belts to separate from the tire carcass, which not only causes the car to pull in a direction but also cause the tire to unexpectedly fail. This is very dangerous and should be fixed immediately.
- Uneven air pressure. If the tires on one side of your car are more (or less) inflated than the other, this can also cause a car to pull.
- Brakes/suspension. If the car only pulls when you apply the brakes, it could be either a brake problem or loose suspension.
- Torque steer. If it only pulls when you accelerate, it could be a problem with torque steer, where more power from the engine is only going to one side of the car.
If the problem is with the wheel alignment, the pull will be consistent no matter what the road is, or the condition/air pressure of the tires, or whether you’re braking, cruising, or accelerating. As you can see, there are many potential reasons why a car might pull in various conditions. We recommend you bring your car in for an inspection to make sure the right problem is diagnosed and repaired.