“A 1911 should not fire unless the thumb safety is off and the grip safety depressed prior to squeezing the trigger. Unless the MRI unit released these safeties, or the gun was in an unsafe condition; either worn parts or kept unlocked with the hammer down on a chambered round, or a combination of both, I don’t see how this could happen.”
If you read the article it explains that the inertial firing pin block was moved out of position by the magnetic field and the impact of the pistol hitting the magnet caused the he firing pin to hit the primer. Firing during drop tests with the muzzle down was why they started using the inertial pin block in the series 80 colt 1911s.