Defective glue. The vast majority of us, including yourself, haven't had the back come off our AWs so it's simply a small subset of it, as is the case with any random manufacturing defects.
I'm not so sure. Perhaps it depends on the climate where you live, how much you sweat, etc? Some have experienced multiple times.
Mine was perfect for 11 months. I would have thought it was a very rare thing and sorta dismissed the reports on here as how they handled it.
However, here I am, 11 months in, and mine did it too after all that time.
Maybe the way I treat my watch it lasts almost a year, maybe the way you treat it, it would last 24 months before coming off, but it seems unlikely the glue is just accidentally messed up on this many.
The camera washer shift on the 6 and 6 Plus seemed to be every single unit given enough time.
I think your statement makes 100 percent sense, but sometimes there is a not so thought out design that with the right environment, shows up, repeatedly, almost without fail.