Stock swap at Grosmont, North Yorkshire Moors Railway

One for the heritage diesels fans. On Tuesday 22/03/16, the 11:00 Pickering to Grosmont left Pickering in the charge of class 37 37264, with 3 blood and custard mark 1 carriages and 2 blue and white carriages. However once at Grosmont, some frantic shunting took place by class 25 no. D7628. The class 37 uncoupled and moved out of the way whilst the class 25 coupled onto the train and removed the 3 blood and custard, depositing them in the carriage sidings to the north of the station, and then bought 2 maroon mark 1's from the sidings and coupled them to the train. The class 25 and class 37 then shunted around to swap places, and the 12:30 service to Pickering then left 15 minutes late. I have no idea why the swap was made.
Camera used - Panasonic HC-V770