Another Hypothesis
I was just remembering that the Captain of AirAsia 8501 had deactivated the flight computer before crashing, possible as a response to AF 447 where the interaction of pilot and computer made the aircraft difficult to comprehend.
Perhaps Lubitz is responding to MH 370 (theory of high altitude decompression), by putting the oxygen mask on after something happened up there. And then it all went wrong.
Is there confirmation that Lubitz is actively locking the door with the switch every time there is an entry code? Or is there no such confirmation?
With all the high visibility reporting and hypothesising of these events, perhaps pilots are trying to learn and respond differently.