That’s true of most airplanes’ doors for humans, but the cargo door on a 75 opens outward. It’s too big to be a plug.

https://kawakaviation.com/wp-content/uploads/2016/12/Kawak-A...

From that very article: “However, since plug doors must open inward, the design is disadvantageous for cargo doors. Due to its large area, the cargo door on an airliner cannot be swung inside the fuselage without taking up a considerable amount of valuable cargo space. For this reason, these doors often open outward and use a locking mechanism with multiple pins or hatch dogs to prevent opening while in flight.”

indeed they do, i don’t have a lot of details in this case, but it seems like it happened at very low altitude. the plug door design works when there’s an actual pressure differential (ie. at higher altitude)