We will never know exactly what happened, but my guess is NB was making her way down the bank to the water for some reason. In this case, you definitely would not want the phone in your hand, the bank is steep enough you would want your hands free in case you slipped. And you would not want the phone in your pocket in case it muffled the call (there has been a suggestion that she listened to conference calls on speaker mode) or to ensure a clear data signal etc. Plus its a quiet rural location, and evidently no one else was around.
If you are about to do something even slightly risky that could result in you ending up in water, pretty much the first thing anyone would do immediately before this is put their phone somewhere safe. It's almost instinctive. To me, phone being found on the bench is totally consistent with NB taking a risk by clambering down the bank for something and then falling. Misadventure IMO.