I can give two reason for your question , the first came from the old prop airliner age, when passenger where approaching the plane (from left) they use to start number 4-3 engine , then 2-1 so right side first.
The second is just for the Airbus 320 , as parking brake recive also pressure from Yellow hydraulic system (N.2 engine), we start first N.2 to permit to rise immediately pressure on Yellow and Parking Brake, and shut down n.1 first , for the same reason.
I wish to say hello at all you , this is my fisrt post...