The causes of gum recession are very varied: aggressive brushing; a traumatic contact on the tooth that overloads it; gum inflammation due to built-up tartar; gum inflammation due to hormonal changes (pregnancy); and disease of the gums and bone – periodontitis. If we cannot find the cause of the recession among these, we may look for it in the fixed retainer. Some types of fixed retainer are made from a material that slowly and uncontrollably moves a tooth or teeth instead of just holding them passively. If the recession comes with a tooth bonded to the fixed retainer having become noticeably crooked, the retainer may need to be removed and the teeth realigned orthodontically. Retainers that carry this risk of shifting are no longer used, but until recently this side effect was not known.
This answer is for general information. Your own case needs an examination by an orthodontist.