This is quite simply the result of very poor weapons training
That's hardly an excuse though. Basic firearms safety is actually pretty simple. The discipline is what is difficult. If you're going to make a living where carrying a firearm is practically a 24/7 activity, there has to be some commitment to acting in safe a manner as possible.
I'm not saying police academies shouldn't hammer these lessons home. However, if the average citizen can pick up a gun and not shoot themselves in the foot with it a police officer should be able to master the discipline it takes to minimize the chances of an accidental shooting.
In the story about this officer, his actions weren't just a lack of following standard firearms safety but went right into negligence.