you are right. Maybe the large sheets were bowed in the center and pressure applied when nailing. Did you start at the top or bottom and work up or down? if you nailed the top and then went to the bottom, you might have made the middle stress out and bow because you created a stress in the middle.
In the future, start at one end and work to the other end, this eliminates the stress and the bow.
Now, your job can be tested by placing you hand with weight to push in and see if there is a bow in the area of your chair rail or more to the middle. If you feel a space between the rock and the studs then you need to fix this. Regretfully, you might have to ease the stress on the top or bottom for the rock to settle down and lay flat. You and I know that that entails alot of work and redoing already laid rock.
I feel that if you pressured the rock to lay flat in a bowed area, you will still have problems down the road.