What causes a garage door to bend in the middle?

A garage door bends in the middle for one of three reasons: the door is undersized for its width, the top panel lacks reinforcement struts, or the opener is forcing a door that is out of balance. Single-car doors rarely bow because they are narrow, but 16- and 18-foot double doors put significant stress across the long horizontal span, especially non-insulated single-skin steel models. The opener arm pulls only at the top center, so without a horizontal strut the steel flexes there every cycle and eventually takes a permanent bow. Broken springs make the opener work much harder, which accelerates the bending. The fix is adding a horizontal U-strut across the top one or two panels and rebalancing the springs.