I kind of suspect the original installer increased the reveal of the shingles so he could cover the roof with fewer shingles and come in with a lower bid. Either that, or he really had no idea how much reveal was required and got it wrong and was not paying attention to the nailing. If nobody ever noticed, it would probably last for a few years. But since his nail gun blew through what was then just a single layer, the shingles started sliding off and it became obvious. What a disgrace of a roofing job.