Jim Weisgram writes:
[...snip...]

I agree. But again I would have thought such "cupping" would occur
more during the wet months when the panel would be expanding against a
rigid frame.

[...snip...]

Hmm, if it was just a change in humidity, the panel would expand
equally on both sides, no cupping.

Perhaps you have a difference in the finish on the back side vs. the
front, so the panel moves more on one side than the other. How are
they finished, front and back?

Both sides are varnished and I have notices this now on most but not
all of my doors (and again these are all interior doors with not
appreciable difference in atmospher on the two sides). The degree of
cupping varies by panel -- some are quite severe, others are less so.

Again the only thing I keep coming back to is that the panel is a
single large but thin sheet of wood so that it may not take much
"differential" stress to lead to such cupping.