Ed, My best guess is too much humidity. Many customers with sharp pianos have "proclaimed" but we don't have water problems in our home. My reply is always, "pianos don't lie" about moisture problems in homes or institutions. Customers have located: holes in window seals , standing water under their homes, broken water pipes, air conditioning vents blowing on pianos, too many live plants in room with piano, water in basements, and finally one lady continually ran a "vaporizer" which blew directly on the piano. The sound board was so swelled, some notes would not sustain a tone. She was highly upset I couldn't fix those notes immediately. She had never heard of a vaporizer fouling up a piano. Neither had I. Carpet cleaners who steam carpets can raise piano pitches substantially. Good Luck, Tommy Black Decatur, Ala.
This PTG archive page provided courtesy of Moy Piano Service, LLC