This is a guess...the valves that are in the showers are not pressure balanced or thermostatically controlled. With that many people, the pressure is probably fluctuating, which changes the hot/cold balance. That is exactly what the pressure balance valve is designed to help prevent, or at least keep from scalding or freezing the person in the shower. Are the old pipes copper or galvanized? If galvanized, they may be on their last legs - it might be repiping time.