Thin original single-layer steel
Those doors shipped with thin, single-layer steel, little or no insulation, and torsion springs sized to the bare minimum for the door's weight at install, not for twenty more years of use. On a door that age, the foam core (where there is one) shrinks and pulls away from the steel skin, the bottom seal hardens and cracks, and the spring - rated for maybe 10,000 cycles when new - starts showing fatigue right around the cycle count a house this age has put on it.
