Gum recession can be caused by inflammation of the gums (periodontitis) or by traumatic irritation (aggressive tooth brushing, for example). During orthodontic treatment, gum recession can happen if the gum is the thin type (or the type that is prone to recession) and the teeth are moved towards the edge of the jawbone. This kind of tooth movement is not desirable, and the orthodontic plan usually aims to place the teeth in the middle of the bone rather than at its edge.
This answer is for general information. Your own case needs an examination by an orthodontist.