Could be sunburn (since leaf is facing window--any really hot days recently?) What are your temps like, and how frequently do you water? Could this plant have dried out too much?
But since other leaves appear to be dying from the base outward, my first inclination is actually root rot. If the roots are gone or damaged, the leaves will wilt/dry, and this has a different appearance than normal senescence. I'd get in there and check em. Is this plant in an undrained bucket of some kind??