I wouldn't be totally surprised if 'leaving the images on the card' for a period of time...weeks/months?...might cause some degradation...a bit here, a bit or two there, going bad. Perhaps it's the result of a low-level emission such as from a CRT or other electronic device(s) in the proximity that caused the card(s) to go bad.
I learned a similar lesson 25 years ago. I left a partially exposed roll of film in one of my two cameras for several YEARS. After finally getting the film processed, the results were major color shifts, fading, etc. Anyone with too-old film knows the story. Even after scanning the few barely-salvageable photos, I still could not recover the colors and detail with my limited Photoshop skills. Unfortunately, the pictures are not re-shootable. They were taken while on my 2nd honeymoon. We've since divorced.
