Why is Madison police chief Nobel Wray retiring?

The report in the Wisconsin State Journal ties the retirement to the shooting of Paul Heenan:

Wray and the department have taken much criticism following the shooting of Heenan, who was intoxicated when he entered a neighbor’s home, then struggled with the homeowner and an officer responding to a reported burglary in process. The department and Dane County District Attorney Ismael Ozanne cleared Officer Stephen Heimsness of wrongdoing in the shooting, but Heimsness later agreed to resign from the department after Wray sought to fire him for unrelated allegations.

The U.S. Justice Department is now reviewing the shooting, and Heenan’s roommates are pursuing a complaint against Heimsness with the Police and Fire Commission.

This is connected to union problems:

Wray has disputed claims by the police union that the department suffers from a lack of communication and leadership and a deteriorated working environment. Those tensions came to light after Wray filed a complaint against Heimsness with the Police and Fire Commission alleging 118 counts of violating departmental policies, many having to do with comments Heimsness made about citizens, dispatchers, supervisors and fellow officers on his squad car computer.

In a July 2 letter to Mayor Paul Soglin, Officer Dan Frei, president of the Madison Professional Police Officers Association, asked Soglin to work with the union and Wray to improve the climate and working culture.

Here's the discussion in the forum at Isthmus (which was on fire over the Heenan shooting). Crockett wrote:

You don't touch the blue line without getting zapped. The article makes it sound like the union didn't like Wray airing dirty laundry.

Green union terrace chair said:

As soon as the story broke about the chief and the police union boss going to the mayor to ask for help in implementing institutional change, I knew this would be the result.