This happens when the heel of the tyre has torn leaving the sidewall unsupported resulting in a sidewall bulge. This is commonly caused by under inflation which allows the tyre to flex on the rim which in turn allows the nose of the rim to chafe and eventually cut through the toe. It can also be caused by poor rim condition. It is common for restorers to cut ‘tidy up’ the nose on the rim edge to remove the sharp corroded edge. This leaves the heel of the tyre unsupported when under load causing the rim to skin off the tyre heel.