Images Darkened After printing

I printed several versions of contact sheets of 181 images over the period of several weeks of editing them.  Each of these print sessions showed up in the histories of each of these images.  Somewhere, somehow, in one of these sessions of printing, the exposure of ALL of these images was significantly darkened.  When I cleared the histories of the images, the images returned to their previous, undark appearance.

So, something I did, or set, in one of these print sessions changed the images - unfortunately changing all of the images.

Any suggestions as to what I may have done to have caused this?  It was rather tedious to go back and delete the print sessions from each of these image's history.