I have a book on the Convair CV880 and CV990, and it shows a picture on one of the pages of an 880 that was converted for cargo-carrying, landing at an airport with the cargo door open. Caption says: Photographer Larry Ivan Potoski was down to his last frame on Sept. 22, 1979 when the unbelievable appeared in his view finder. Monarch's N8816E was returning to Miami International with its cargo door fully open, proving how good the door hinges really were. This turned out to be number 38 on a 36-exposure roll of flim.
Basically, what must have happened to the DC-8 and the Convair is that the cargo door was not properly latched, and swung open on its own. The lack of total pressurisation at sea level vs. cruising altitude would allow the door to open outward if not properly latched. Thankfully, there were no severe consequences to either the CV880 I mentioned or the DC-8 in the photo.