Open diffs split the torque EQUALLY between the wheels, with total torque dependent on the wheel with the least traction. That's how they work. I've explained it a million times, lol. Search it out.
If one wheel loses all traction, the other wheel gets the same torque as the slipping wheel, and thats when you get one wheel spin. But there is no mechanism in a differential to always send the power to just one wheel or the other -- though it can seem that way.
But if both wheels have equal traction, they can both get spun when they break loose.
You have more torque to the wheel in reverse, your lowest gear, and the cut of the gears makes conditions a little different. In addition, when you accelerate in reverse, the weight transfers to the front wheels, allowing the rear wheels to break loose more easily. Combine the higher torque in reverse with the weight transfer, and you make it easier for the wheels to both break loose before one breaks loose and "bleeds away" any torque.
As for a further explanation of how a differential works -- look it up!