I guess checking your doors was not on your pre flight checklist the two times your doors were not closed?
You guess wrong. It is. And I did. This happens to everyone on small GA aircraft at some point. 90% of the time the doors latch fine when you close them. But once in a while they don't latch and you can't tell until vibration or air pressure pops it open. The situation has nothing to do with forgetting to close the door (usually).