Were you charged with any driving offense? Was the other driver charged with an offense? If only one driver was charged, it is usually that driver that is considered to be at fault. Additionally, as others have mentioned it is usually the driver in the rear that was at fault. The fact that it was a passing lane isn't really relevant since you are only allowed to pass when it is safe to do so, obviously not in this case. The situation you describe is a classic example of a frustrated driver zipping out and accelerating hard because he was annoyed by a slower driver.
