Have you ever eaten a watermelon and then decided to play with the pips by placing one between your thumb and forefinger and pressing down? If you have then that is kind of the nature of the forces at play in "ejecting" the "rock door". The doorway is so perfectly formed because of the nature of the rock structure and cleavage planes/structure you can see, both deep inside the doorway towards the rear roof crack and, as an example, the sort of almost perpendicular cleavage plane/cracks that can be seen outside the doorway on the left of the image. These are interspersed throughout the rock and are being acted upon by geological forces as well as Mars gravity and act to deposit, form and move the rock and eventually to it being flung out / falling out.
Anyway ... the doorway is too small for even
The Greys that live on Mars
Best
JA