I don't think it has anything to do with improper drying. When cut at the mill, the internal stresses in the board itself were neutralized and remained flat. Once you cut it into small strips, the stressed areas moved to relieve the stresses.
This is just my best guess.
The solution is largely dependent on what you are going to do with them.
Tony B Retired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B