I'm presuming that it was a Glock, in all probability. Do you know something else, like the cop had a revolver?
One round is a negligent discharge. FOUR rounds, on the other hand...................
Maybe not a Glock, but it's quite likely. He might have pre-cocked a DA first shot pistol, and then under stress he put some weight on the trigger, as if it was still going to require a DA pull.
This used to happen with revolvers a lot. Riots in Miami broke out in the mid 80s when an Hispanic cop had a black kid spread over a pinball machine, prior to cuffing him. The cop cocked his DA revolver, and when the kid allegedly moved: "BANG!" A lot of police depts removed the SA sear from their revolvers after than, making them true "DA only" guns.
Now, with 50 different auto pistols in use, it's a real bag of worms. Cops may be using SA only (1911 type), DA only, DA first shot followed by SA follow ups, Glock "safe action" which is close to SA, and so on.
If cops are transitioning from one type of pistol to another with a different trigger type, you can have real problems. When that "muscle memory" kicks in, and under great stress your reptile brain think it's going to take 12 pounds to make that BANG and it takes only 4......
This is not "thought out," that's the point. It happens on an almost unconscious level. This is why much training and uniform policies are super important.