I’ll make no comment on exactly what happened here, but I can relate a personal experience that was one of those learning moments.
I was alone, taking off from a field in Arizona in the middle of nowhere. I had landed at the field with a friend who did a very expedited runup, and not wanting to get left too far behind, I expedited my runup and takeoff checklist as well (yes…I know).
I taxied out, applied power, and had a very normal takeoff roll and takeoff, until I hit about 400’, when the door opened…it had been latched, but not locked.
Without panicking, I reached over and tried to secure the door…one, twice, three times. I failed. I turned my head away from the door to make a call that I was returning to the field, when I realized that I was now looking at the desert floor.
After reestablishing a climb altitude, I returned and landed, secured the door, and left uneventfully.
The response to my radio announcement that I was returning to land was embarrassing. “I didn’t think that was going to end well. I don’t know what you were doing inside that plane for a couple of seconds, but your instructor definitely taught you better than that. Fly the airplane.”
I replied with “Door issue I should have ignored. I’m sorry you had to watch that.”
Update: I should point out that the lesson stuck. On another flight, in another plane, a broken latch on a baggage door resulted in the door opening, again on takeoff… making a racket. On that day, the instructor jumped a little, and started saying what is making that noise. I responded with a quick radio call and then told him the plane was flying great and we’d sort it out. Took another couple of seconds to realize it was the baggage door. Fly. Live. Learn.