If every American was able to participate in the Tueller Drill, which to the best of my knowledge is still used for instruction at most police academies, this question would never come up again. . .:angryteet
It was also used in my NRA instructor training, and I in turn use it in my carry permit class. As an instruction tool I feel that it's invaluable. People often believe that carrying a firearm is akin to magical armor, and the drill can bring home a slew of valuable points.
As far as the magazine "falling out", there are multiple possibilities, including the officer releasing it by accident. I'm not implying this is the case, I don't know any better then anyone else here. In stress, people do all sorts of things, that seem silly in hindsight. Luckily he had backup.