My impression of "creasing inwards" is that the wood is swelling, not shrinking. In which case it would be because the humidity has increased. If that is the problem, wetting the wood would make it worse.

That's a possibility as well.
Dampening/drying thin wood is completely reversible, in my experience, so a quick experiment with a damp sponge and a hair dryer/heat gun on 'Lo' should tell the tale.
As you imply, over-enthusiastic sanding -especially if the strips are thin - could cause problems.

If there is 'creasing' at a glue line, does this indicate a failure of the glue bond between the strips? Perhaps this is another argument for trying Bjorn Thomasson's 'strip dry, then epoxy' technique? (I used that method on my last build and it worked well. OTOH, I don't recall having many problems with failure of the strip glued joints in years past, either.)