Why is parking brake set OFF before jacking?
The wheel is removed with the PB depressurized so that the discs are free to float (both rotationally and slightly axially) when the wheel is being slid off, to prevent any potential binding of the disc projections where they engage the slots in the bore of the wheel as the wheel is pulled out. If the discs are locked in place by brake engagement, any off-axis movement of the wheel unit as it's being slid off could jam it on the discs, making it really hard to pull off.

When the procedure is started, the airplane is chocked at the other gears, so the parking brake is already made redundant at this point.  As a former aviation technical writer in another life (not Airbus), I would say it was done that way for work flow purposes (getting all the cockpit related precautions out of the way before you start the procedure) because you don't need the PB on once chocked, and you're going to have to release it anyway, so it's done during the preparation steps prior to the actual jacking.