I find that oak does that often for me, but it is difficult to control. We sometimes forget that wood is porous and 'breaths' and the environment around us can affect it tremendously. I used to have an antique Singer sewing machine that was my grandmothers, and every summer the top panel warped. Each winter however, it would dry out and be flat as a pancake. I know now that if I sealed it properly in the winter, it may have helped prevent the summer warping. (I lived in the Chicago area then, where the summer months were very humid)
Perhaps you need to seal things better. I realize this is hard with fretwork because of so much open end grain. Maybe using oil and letting it absorb would help block the pores from pulling in additional moisture. I like to oil my fretwork pieces, wait a couple of days until it is absorbed, and then spray with shellac. While it may not help the pieces you already have, it may help in the future.
I hope this idea helps a bit.
Sheila
