If you have story progression enabled, and the game thinks the town is overpopulated (which may happen if it created sims to fill roles or move in), it will kill off sims to lower the amount of them in town. Usually it picks sims the active family doesn't have a relationship with. You will see stories in the newspaper about these sims drowning, starving, or getting struck by lightning. The game just kills them off and puts the headstone in the mausoleum, but it was probably not an actual drowning animation or anything that you could have witnessed or stopped.