I know it was almost 2 years ago, but I hope you're still around here.
I had an unusual experience today, similar to yours. I reversed my Mini on approx. 50 yard long, and the power steering suddenly failed in a strange way: it felt like if it was doing it's job into the opposite direction, making the steering almost impossible (I'm not _that_ weak, and I had cars without power steering). Stopping and starting the engine solved the problem ( a'la Windows '95 ). Perhaps the reversing confused the software via a supposedly one-way sensor?
Did you found the cause of your problem, or simply replaced the pump?