I bought a used Stumpjumper with Alivo/XTR components. Interestingly, this bike has the rear derail. shifting reversed. I.e. the upshifter downshifts and vice versa. The front derail. seems stock. Easy enough to fix but I'm trying to figure out why they did this .. is this an off-road thing (e.g. downshifting easier into a turn)? Anyone do something similar or maybe the mechanic just made a mistake?

Sounds like a rapid rise (no tension = chain defaults to the low gear close to the wheel). You'd need a high normal derailleur if you wanted to switch it.