Fallbrook sits in a mild temperate band, so the usual coastal culprit - salt-driven rust - isn't the main thing we see on panels out here the way it might be closer to the water. What we see instead is sun. Steel panels that have sat facing west or south for forty years develop a slight bow or warp from heat cycling, especially on the thinner gauge steel common in 1970s-2003 construction, which covers most of this area's housing stock.
Fallbrook's median household income, at $87,293, runs lower than the roughly $110,316 median across the wider service area, and it shows up in the kind of call we get. More homeowners here ask for a single matched panel instead of a full door replacement - it's the cheaper fix, and on a door that's otherwise sound, it's often the right one. We won't talk anyone into a full replacement when one bent panel on an otherwise solid 1980s door doesn't call for it.
The other driver is simple impact - a trailer, a bike rack, a car bumper catching the bottom section. On original hardware that's never been stress-tested by a repair before, one hit can crack a panel that's already brittle from decades of thermal cycling.