Doesn't matter if he was experienced with guns as it could all chock up to 'mental fog'.
In the released documents it is noted that within the 47 second long video as he walks up to the girls, one of the girls states, "he's got a gun" and that the video also recorded him chambering a round can be heard and him telling them, "Guys ... down the hill".
It's yet to be seen if there was a struggle at the crime scene, but I suspect that something occured that caused him to c.o.c.k. (edited as it took the word out as 'advertisor censored' -- sigh) the slide thus ejecting that first round and chambering another to get the girl(s) back under control.
Perhaps he simply forgot that he had already chambered one on the bridge and in the heat of the moment of whatever was occuring at the time failed to notice the original round ejecting.
From my experience with the P226 (it was my personal weapon in the Army), I highly doubt the ejection was caused by one of the girls struggling with him or kicking the weapon as that most likely would have led to his finger pulling the trigger if his finger was within the trigger guard; certainly not cocking the slide.
With 'heat of the moment', I liken it to "fog of war" ... where sometimes so much is going on around you that you forget something that should be obvious (like having already chambered a round). Even with the pros, fog can happen. Even worse when one forgets they have a round chambered and pulls their trigger resulting in an accidental discharge (called a Negligent Discharge in the Army and very court-martial worthy). Hopefully no one gets struck by that round best-case scenario. Worst case: it kills someone (it's happened). Muscle Memory.