No knowledge aside what I read in the Seattle Times, but educated guess is that someone came up with a different (easier) way to ground part of the system. The 'continued airworthiness' folks do periodic audits to make sure things age gracefully in-service (this I do know first hand - was involved in some of it years ago) - and they may have discovered that while the grounding path was fine when new (i.e. functional testing), it might not be after it got banged around in service.
Probably worth noting that this was apparently self-reported by Boeing. Maybe they are learning...