A cable that's original to a 1978 door has had over four decades of drums turning against it, and galvanized cable from that period wasn't rated for the number of cycles a door sees today, especially on a two-car door that gets used several times a day. Rust at the bottom fixture is the usual starting point - moisture works into the cable strands where they clamp to the bracket, and it corrodes from the inside where you can't see it until it lets go. The climate here doesn't bring hard freezes, but it does stay damp for long stretches, and that steady moisture does more to a cable over 40 years than one bad winter would. We also see cables that were never the problem to begin with - a bent track or a roller that's binding puts uneven load on one side, and the cable on that side wears out years before the other one does.