Why does my 2012 Ford Ranger's steering wheel move on start up?

This is a strange one. If you had a post-2015 Ranger, the suspicion would be that the electric power-steering was playing silly games and turning the wheel to the left randomly. (Electric power-steering with the ability to move the wheel without driver input is the technology that has made driver aids such as lane-keeping assistance and self-parking possible.) If there’s a fault with the electric motor, the computer that controls it or the sensors that tell the system what’s what, then you can have a problem like this occur.

But Rangers built prior to this date had conventional hydraulic power-steering which, without driver input, should not be able to turn the steering at all. Except yours does.

Probably the most important piece of advice is not to drive the vehicle any farther until the problem is fixed. Obviously, having a steering system with a mind of its own is a potential source of disaster.

The actual problem could be do with the valving that controls the steering rack (and its power assistance) and a pressure imbalance between the right-hand and left-hand side of the steering rack could potentially cause this sort of problem. The valve that controls the speed-sensitive aspect of the steering is a likely culprit here as a faulty one can lead to uneven pressures within the steering rack.

If you want to be proactive, check that the power-steering fluid in the pump (under the bonnet) is at the correct level. Then, start the car with the transmission in Park and turn the steering wheel from full right to full left lock a couple of time. This should purge any air from the hydraulic fluid and restore balance to the system. But really, this is a potentially very serious problem and needs to be checked by a specialist.