Treated lumber, which is notorious for warping and bowing, is treated by dunking the lumber into a vat of liquid and then applying pressure to force the liquid into the wood. That means the wood is very wet and generally arrives at the lumberyard wet. Wet wood warps very easily. There are a few simple ways to counteract this tendency.
How to Prevent the Warping of CCA Treated Wood
Work with the wood before it dries. If you use the wood while it is still damp (and therefore straight) you can secure it into place before it dries, and it can dry in place in a straight manner.
Clamp the wood. A similar method to the above would be to clamp a piece of wood to a workbench and allow it to dry that way. This would be helpful if you needed only a small amount of lumber for a small project or if you could not use damp lumber.
Use screws, not nails. When wood dries and warps, it produces a lot of force. That force can cause nails to raise. However, the force is insufficient to raise screws, so if you use them, the board will dry in the straight position.