Early booking worthless

On 26 July 2015 my wife and I were booked to fly Kulula from ORT to George at 06:50 flight MN903. 24 hours before I checked in online reserving seats in row 2 (d and e, I think). At the boarding gate we were told our seating had been changed to row 27, the very back of the plane. Our prebooked seats were vacant and the aircraft half full. No explanation given.