Why suddenly are my cupcake papers separating from the cake after they have been cooled for several hours?

I hadn't heard of that happening before but found this online...

Why do the paper liners separate from my cupcakes after baking?

Some suggestions:
1. Use plain metal pans. Do NOT use pans that have non-stick coating.
2. Do NOT use any cooking spray on the pans or the liners.
3. Try to use plain white paper liners. Some liners (especially decorated ones) have a waxier coating on them, making them slip off more easily.
4. Also, a reader suggested using Reynolds-brand foil liners that come with paper inserts between them. Use BOTH when baking the cupcakes. The foil liner will help hold the paper liner in place.
5. Only fill the pans HALF-FULL with batter. Any more than that, and the batter will spill over the top, sticking to the pan.
6. Allow the baked cupcakes to cool in the pan for just a few minutes (approx. 5). Then remove them to a wire rack to cool completely. If you leave the cupcakes in the pan too long, steam will condense on the wall between the pan and the cupcakes, loosening the paper liners.

This was another response to the same question...

I've experience the same problem . . . sometimes they separate and I have never been able to figure out why. Now to avoid problems I bake in plain white papers, frost and decorate them and then drop them into a fresh paper liner. They actually present better that way anyhow, especially when I'm using decorative printed papers. I have had conversations with several bakers who have also had this issue and no one seems to agree as to the cause . . . one blames the paper liner, another claims it has something to do with creaming butter/sugar and still another thinks the cooling process is the issue. I do hundreds of cupcakes each week and can find no common thread as to why . . . now we use the second paper on everything.