We had the same problem with WD Greens. We'd write a backup to the drive in a dock each week, then store the drives as monthly backups. About 6% of the drives failed when we plugged them back in.
I'd be more inclined to blame damage during handling, but only because I cannot think of an alternate explanation. There is no constant refreshing of data that happens with a disk drive (to my knowledge) such that a drive being left plugged in should be more reliable than one on the shelf.
