If the boards warped almost as soon as they were cut, it is stress in the wood. If the wood was not dried properly, it can have quite a bit of stress in it. Also, internal stresses caused by limbs can be a factor. If you cut the 4 ft lengths and they were warped the next day, it is most likely because of moisture content. The moisture content of the board in the middle can be different than on the outer edges. When you expose new wood to the air the moisture content will seek to acclimate to your shop and can cause warping again. When I cut any long pieces, I always sticker, stack and weight down the shorter pieces for a day or two in order for them to settle down. I learned this the hard way when I first started ww. I had milled and cut several boards for a project and when I returned the next day they were warped all over the place.
