All posters have valid points, but Larry gets my vote. I once had a student rip a 2x4 in half. She stood it on end and left it for the day. The next morning the 2 pieces looked like corkscrews. I wish I had a picture. From end to end of a 8'er there were at least 2 revolutions. I know you are not using construction grade material but it makes me wonder--could it be reaction wood?
"Individual commitment to a group effort--that is what makes a team work, a company work, a society work, a civilization work."