Well, these appear to be reverse threaded props and self tighten during flight. It appears that the mavic shuts the motors down hard and fast and the forward momentum is enough to unscrew them all the way when the motors stop???
EDIT: I twisted the props a tiny bit tighter on the next flight and this did not happen again.
I'm happy you figured it out! your lucky they didn't fly off while braking during flight if they were that loose. If i was going to explain how tight the props should be i would say as tight as you would the plastic cap on a soda bottle.