There was something about how the bike was stored that exposed the rear tire to more heat and/or sunlight than the front. Also don't discount the thought that the "new" tires were not that new at the shop. Or that the rear tire was displayed on a rack that exposed it to a lot of sunlight for a few months before you bought it. But what you see is ONLY from age and exposure to bad conditions. Nothing you did while riding contributed to the present condition.
Anyway whatever the circumstances that tire is toast. It's holding air so it's OK to ride around on in a casual manner but you'll want to get fresh tires very soon. Like the next time you ride it you might want it to be to the bike dealer to buy the new tires. If you tried to actually ride that tire in an aggresive off road manner it's likely that the knobs would first off not have the same traction as a new tire on hard surfaces and second that the damage to the rubber would allow a lot of the knobs to tear out of the carcase.
And since both tires saw the same storage conditions for the last two years don't expect the front to be much better. Even if the surface checking hasn't started yet if you rub the rubber it's highly likely that it'll feel slippery and plasticy hard instead of soft and grippy. Again this is part of aging in a hot environment such as sheds or garages so often get in the summers.