Why on earth would any sane person wish to work after retirement? I retired at 58 and was lucky enough to have 10 years with my wife before she died. The day I retired I vowed never to work again. I never have and never will.
Perhaps they haven't adequately prepared for retirement. Or their nest egg has been hived away by GFC or bad financial advice, embezzlement, or similar. Perhaps their psyche just needs it.