Actually, what we are seeing is the more vulnerable blue strands (more sun absorbed?) failing under extream winch loads, while the core hangs tough. The core is Dyneem, still strong as heck even as the cover fades away. Only the cover between the clew and the winch--about 6 feet--is affected, since is the only portion that sees high loads, depending on how much the sail is reefed. This is not chafe, since this section cannot reach the winch or any chafe point.
Why is the load not simply carried by the core? I suspect that it is for the most part, but that the cover is creeping on the core just enough to have caused this odd failure pattern.