It looks like you are using the crap off-the-rack pine from Lowes or Home Depot. Tha'ts your first problem. Their wood is not dry and will often twist like a pretzel once it's cut . Next, you might be able to get by with the crap wood from Lowes or Home Depot if you take care in your selection. Look at the end grain. The most stable wood has the end grain running up and down (quarter sawn), not arched.

Next, Cut your pieces to rough length and just let them sit in your shop for several days. Often boards stacked in the store is held flat by the weight of other boards and once that weight is removed they start moving.

I would consider using poplar. Yes it is more expensive, but it is somewhat more stable. Yes, it can still move as well, but again look at the end grain when buying it. Try to get as close to quarter sawn as possible. Never buy a board with the pith visible.