Notches that form on your teeth at the gumline are known as abfractions. This specific type of cavity is caused not just by decay (in fact there is often not decay present!), but by brushing your teeth too hard and teeth grinding or "bruxism". It's thought that grinding and/or clenching of the jaws contributes by causing a slight flexure at this fulcrum point of the tooth.
While this tooth seen here was easily treated with a simple filling, in some cases significant gum recession can also be part of the problem. Gum-replacement grafts might be needed to cover up sensitive exposed root surfaces if recession has progressed far enough. Good news is that one easy visit is sufficient for most abfractions!