If it was a system failure then fair enough. This to me smacks of complacency. When you secure the door, there is a little clear circle viewing hole to check that the door has locked. Even if it's partially locked it wouldn't have popped open until the aircraft pressurized. I was shown pictures of this, and the door is wide open just after rotation.
Ever see the story about Hawaiian 811, where the cargo door opened by itself? FAA, Boeing and Airlines denied it for years, until one happened on the ramp, a 747 cargo door opened and no-one was around, except the people who saw it. No-one came near the plane, it just shorted out and opened all by it's little lonesome. Then the NTSB report was finally changed to reflect what actually happened in Hawaii.
Assume nothing. If you get in the habit of assuming things, then when you get into an emergency situation you will not allow yourself to consider all options because you will have already figured out what the problem is and be blind to what the real problem might have been.
And in the case of this incident, don't assume that a person did their job.