I agree, it is a moisture problem with one side dryer than the other. Leaning it against a wall or put a clamp on it so that you can stand it on edge on the work bench or simply jack it up off the bench with small squares of wood or using "bench cookies" or "painters pyramids" so that air can circulate on all sides. I too will spray or wipe some water all over it with a rag or paper towel before setting it up to dry equally allover. I suggest you leave it to dry for a day or at least overnight.
I am in the process of making a head board for my granddaughter's bed and have four pieces of popular between 48" and 60"long with a twist in every one of them.
I've had them standing on end for two days now and there is very little change.
Now, there is a problem for you! LOL
Rog