At least some instances of this bug are caused by the new develop settings added by CC 2015.6 for the Guided Upright tool (2015.6 was released 6/8/2016). When 2015.6 or later first renders a photo at 1:1 zoom that had been imported by 2015.5 or earlier, it adds those develop settings before rendering. Then it compares those develop settings with the old, notices they are different, and incorrectly marks the photo to be republished.
Sometime photos in my published collections began to randomly mark themselves as modified to republish. Photos I haven't touched in years, in galleries I haven't recently changed, all of a sudden appear under 'Modified Photos to Re-Publish.' If I even scroll through a collection, dozens of the images begin to jump up to 'Modified Photos'. I can select the photos and send them back to 'Published' with 'Mark as Up-to-Date,' but then more immediately jump up to Modified.