steering wheel goes slightly but noticeably to the left?

When there is a driver, front passenger and passenger side rear passenger riding the steering wheel goes slightly but noticeably to the left(note the car does not pull to the left the wheel just goes left of center).If I have the rear passenger move to the center or full left the wheel returns to dead center. Also, there are approximately 50 lbs of items in the trunk evenly spread out and drivers' weight is about 175, passenger 200 rear passenger 200. Wheel position normal with just driver or driver and front passenger.