I'd be more inclined to say he loaded a mag and seated a round, but didn't get the mag seated quite deep enough or hit the button on the wrong side to release the BCG forward or similar, then lost the magazine.
I would think a loaded chamber is not routine for a rarely used rifle and in the hands of a highly trained cop person.
Also, whoever hit crook in the back sure hit him hard or alot all at once or with a really big gun. Just based on teh way he slammed down stiff and slid a bit.
Nemo