It is easy for wood to warp in the winter,especially if it is over a heat register.
What type of stretchers? Mitered glued and stapled at the corners? Or the ones with adjustable keys?
If the key type then they need tightened at the corners for starters but still may not be the problem.
If fixed/glued corners I would take it down and lay it flat on a table and see if it is warped? Looks like it already is.
With care you can twist a little warp out of these things,best done at the shop. When I've done it in the past it was before stretching.
Not sure if I would instruct a customer to try that!
After checking for flatness on a tabletop and its off less then 1/4" I just put a 1/4" clear bumper in that raised corner so it will not rock on the wall.
Most if not all of these semi fixes should be done by you. If they live nearby have them bring it in.
Worth a try and a lot less cost then making a new one.
As far as what caused it,good question. Stretchers may not be properly kiln dried? Then it gets in to a home with extra dry heat and there is the recipe for disaster.