Here's a link to that paper discussing wood warping during drying:
UMass Amherst: Building Materials and Wood Technology » Do I Lay Decking Bark-Side Up or Down?
Basically, if a living tree had a round trunk and all the growth rings were concentric, then no matter where you cut a board out of that tree trunk, the growth rings would tend to straighten out as the wood in that board dried out.
So, if you cut a board horizontally from the top half of the tree trunk, as that wet wood dried, the board would cup upward (concave up) so as to hold water on it's surface as it dries out.
Similarily, if the board was straight when it was dry, then wetting it will cause it to warp such that the growth rings form a smaller circle.
Theoretically, lumber isn't supposed to be cut to size until after it's kiln dried, and "kiln dried" means a moisture content of 19 percent. But, you need wood to be truly dry before you can pressure treat it well, so maybe your pressure treated lumber was straight when it was installed, but warped when it got wet. Typically, tho, once wood is nailed into place, it doesn't warp. Mechanical fasteners like nails and screws are plenty strong enough to keep the wood from twisting or bending, and once it's dried in place, it'll retain that same shape.