We seem to have a manufacturing or repair defect here.
Off topic, as for DC9 vs. B737, there were different design philosophies at work. Douglas was more elegant than Boeing, or thought they were, as anyone who has examined the pilots opening windows on both aircraft will know.
However Boeing didn't design that elegant tracery of linkages for the DC10 cargo door either.
My experience of dealing with Douglas was always: "Fukc you! What would you know? You are just an airline engineer! We built the DC3!" Boeing actually had a certain humility in dealing with its customers, at least on an engineering level.