Why (ending spoilers)

The Chief ruled the town with an iron fist and the cops all stuck together under his rule. In the end when Sims takes out the Chief’s vehicle, maybe the other cops know his reign is over and they are finally willing to do what is right, without worrying about being taken down by the corrupt Chief.

When the chief shot "Serpico," camera panned and showed reactions of some of the other cops, who looked worried and confused. In the radio communications at the end, other cops asked if they still had an officer on board. Meaning is Evan Marston aka Seperico, still on board? The chief responded by saying, "no, we have 3 suspects on board." So they are smart enough to know where this is heading, they just needed a new leader to help them do the right thing, so they followed Sims when she pit maneuvered the Chief. After she knocked him out, the radio communication was also very matter of fact, an officer asked, "You got the chief?" Like they weren't surprised. She then acknowledge, and they then confirmed they will escort the "suspects" to hospital.

I understand the confusion, me too. I thought there should have been more dialogue of Sims quarterbacking the "coup" to spell it out for the audience. As is, it forces the audience to make too many assumptions or guessing about how and why these officers decided to "escort" the suspects to safety. For example, "This has gone too far, I'm taking out the chief.. you boys make up your own mind." Something like that. LOL! As is, it was very subtle. If you watch it over again, and listen to the radio convos again, it will make more sense.

It's called "deus ex machina". It bothered me too.

The black lady changing her mind, I can buy it.

also, the dispatcher on the radio states that the state police were 2 minutes out, i think that helped to sway the other officers in the right direction