Tabletop was flat after glue-up, then cupped a week later

I glued up a small solid-wood tabletop and it came off the clamps flat enough that I felt very pleased with myself. A week later there is a noticeable cup across the width. Not catastrophic, but enough that a straightedge makes the optimism disappear.

The boards were stored in the shop for a while before glue-up, although I did not measure moisture. I alternated the boards mostly for appearance rather than trying to force the growth rings into a pattern. After sanding, I put finish on the top face first because I ran out of time. The underside stayed bare for several days, and the weather also became much more humid that week.

I have read enough conflicting advice to know that 'always alternate growth rings' is not a magic spell. What I am trying to understand is which variables matter most now. Did the unfinished underside create a temporary moisture imbalance? Was one board simply wetter than the others? Is the cup likely to relax if both faces reach the same conditions, or am I looking at material that will need flattening again?

Before I attack it with tools, what would you check or wait for? I would rather learn what moved than make the tabletop thinner every time the weather changes.