Being a MYOG nut, there are rolls of material that sit around the damp basement for eons.
Have often wondered why some PU coatings delaminate after a few years, and others not.
Two delams that come to mind are a never used TNF bivy with a built-in pad, and a Eureka tent that was heavily used for over five years at least.
On the other hand, I have some old TNF 1.6 oz PU coated nylon that was used on their lightest pup tent decades ago, and there is still no delamination (TNF actually used to sell fabric).
There is also the situation where the PU gets gooey - no idea what's going on there either.
Best guess is that it's the quality of the coating and its application, which could vary with the same brand, depending on what supplier is doing the job. But poor storage if a factor pointed out in OP Benjamin's recent thread: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=81746
+1 on the use of the term, "defective." If coating won't stay on in ordinary use, with dry storage at reasonable temps, it is probably that, making a good case for a warranty issue. But as was pointed out, we don't know how the pack was treated. Only the OP knows that. "Let your conscience by your guide," OP. -Jiminy Cricket.