The thief's been watching too many movies. Notice that he immediately racked the slide after getting the gun in his hand.
Racking the slide when you acquire a foreign weapon that you may need to use immediately is pretty standard procedure in real life. I know some instructors who teach racking the slide on a gun immediately after a disarm, for example, which is essentially what happened here (albeit the disarmer was the bad guy).
Edit to: forget the whacking the magazine on the base before racking the slide above.
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