There are a number of possible causes of this.
One can be changes to metadata, not images (and you have some control over this in each publishing setup). These changes can in fact be indirect, for example changes to a keyword itself can cause republishing of every photo with that keyword attached. Some plugins (Smugmug's own for example) will be smart about the republish and send only the metadata, though you cannot see this in the published folder display itself.
There was also a bug/feature in 2015.6 (I think -- the one where they introduced the adjustable upright features). This resulted in a different set of develop data internally when it was used, giving a different digest and in turn marking it for publish. The bug/feature was that if you did NOT have a preview built for an image before the upgrade, building the preview would change the develop data, which in turn would mark the image to republish. You could quite literally sit on a published folder, and as it would build previews watch one by one images flip from published to to-publish. Supposedly this was addressed in the next version, though also adobe never really acknowledged what I saw (their comments were all Mac related but I saw this on Windows). I did a build of standard previews of all images, then marked all as published (as they were when I started), but this was VERY tedious as you have to do it by published collection.
The other possibility is that the publishing is not being successful, though you can probably see this. Some of the online publish plugins can get errors, leaving the image still marked for publish on the next run (as opposed to retrying in that publish run).
And of course changing image data, as you would expect, can cause it.