Well, you train cops to dominate every situation and add to that the growing attitude of cops that it is Us against Them, where them includes everyone who isn't a cop and you can get this type of situation.
The uniformed cop sees someone who he thinks is one of Them impersonating one of Us and goes into action.
Both are not going to let anyone but themselves dominate the situation and that just fuels the uniformed cop in to his belief he is facing one of Them....The Enemy.
What is surprising is that they didn't shoot each other...but it appeared to have been close.