Originally Posted by onenorth

Going on your theory, if the loose bolt was on the outboard side of the hinge bracket then there would only be one other bolt holding the door to the bracket on that side. Pressure would cause the door to move outward and twist against the remaining hinge bolt, causing it to shear, then the other two bolts on the inboard side of the bracket would also fail as the door pressed away from the bracket. The intermittent pressure warnings could have been the door bowing outward against the one remaining outboard bolt and breaking the seal. Maybe.

Originally Posted by chrisl137

I suspect three (or even one) of the retaining bolts is strong enough for the little load they see, but my speculation until a few minutes ago was that the same person installed all four wrong and when the last one let go and the plane had a bump, the door departed the aircraft.

Now seeing loose bracket bolts in the UA pics it could be worse, with a bunch of those coming out. If the guides are missing from the door, it would be those bolts that came out.