I have not edited the photos. I do not apply adjustments on import except for whatever adjustments Lightroom applies when I haven't modified their default settings.
Please disable use of Graphics Processor in Lightroom
Go to Lightroom>>Preferences>>Performance and uncheck "Use Graphics processor"
Then reimport a batch to a new catalog and see if the corruption repeats.
they *all* looked good
Initially you were viewing the thumbnail embedded in the raw image file.
Then Lightroom starts building previews from the NEF raw data. (which may be corrupted)
Possible causes for the corrupt 'raw' data-
The camera card is corrupt- raw image data is corrupt?
The Camera-to-computer download has hardware faults- bad USB cable, poor USB plug connections?
Your hard-drive is beginning to fail and images saved are not written to disk correctly?
If you used 'old' versions of Nikon software (NX2?) then this software has been known to corrupt image files.
Re-importing your images may not fix the problem. If the camera card is OK then importing again may be OK if all hardware (cables, etc) is OK. Try using different methods or different card reader and cables.