Not later than 15 days after a general verdict or the filing of the decision or order, the court upon its own initiative may order a new trial for any reason for which it might have granted a new trial on motion of a party. After giving the parties notice and an opportunity to be heard on the matter, the court may grant a motion for a new trial, timely served, for a reason not stated in the motion. In either case, the court shall specify in the order the grounds therefor.