Most likely the acid in the wood stain than any direct contact with sunlight. I don't claim to be an expert on the aging of fabric (far from it), but if the culprit were sunlight I think you're fabric would of gotten paler instead of darker. At least that's what happened with my ugly leopard print fabric that's used to cover the window of my bedroom...used to be an ugly brown, but now its an even uglier yellow because of the direct contact with sunlight. So I'm guessing your fabric got brownish because it absorbed the acid and kind of took on the color of your wood stain.