What causes a garage door to bend?

Garage doors bend most often from impact damage, where a vehicle bumper, lawn tractor, or basketball backboard strikes a panel hard enough to deform the steel. The second most common cause is operator strain, where the door tries to close on an obstruction or a broken spring leaves the door so heavy that the opener bows the top panel as it lifts. Wind load on a poorly reinforced door can buckle panels in storms. Older single-skin steel doors without insulation or struts are especially prone to flexing in the middle as they age. Improper opener installation that places the trolley arm at the wrong angle puts continuous stress on the top panel and slowly bends it over years of use.