It was straight and square when it was milled. Problem is that the big box stores cut corners during the kiln drying phase, so the boards have a ton of moisture still in them. As that dries, the boards twist and cup.

Plus poor protection from the elements at each storage step, which means rapid temp changes, which also causes wood movement.

Go to a local lumber yard. They tend to do a better job at kiln drying. You’re still going to have warped boards, but far fewer in my experience.