Looks like he got the once over for having campaigned for HRC when he is not allowed to do so.

The Hatch Act explained by OSC (Office of Special Counsel)
The Hatch Act, a federal law passed in 1939, limits certain political activities of federal employees, as well as some state, D.C., and local government employees who work in connection with federally funded programs. ​The law’s purposes are to ensure that federal programs are administered in a nonpartisan fashion, to protect federal employees from political coercion in the workplace, and to ensure that federal employees are advanced based on merit and not based on political affiliation.​​​​ ​​

Here's CNN's take on it:

Housing and Urban Development Secretary Julian Castro violated a federal law against mixing official business with politics when he touted Hillary Clinton in an April interview with Yahoo's Katie Couric, the Office of Special Counsel said Monday.

The Hill:

HUD's Castro violated law against political activity

Housing and Urban Development (HUD) Secretary Julián Castro violated a law that prohibits federal employees from making personal political statements while appearing in an official capacity, according to a new report.

The U.S. Office of Special Counsel (OSC) released a report on Monday concluding that Castro, considered a possible running mate for presumptive Democratic presidential nominee Hillary Clinton, violated the Hatch Act during a Yahoo News interview with Katie Couric on April 4 by sharing his views of the presidential candidates.

“Secretary Castro’s statements during the interview impermissibly mixed his personal political views with official agency business despite his efforts to clarify that some answers were being given in his personal capacity,” the OSC report said.

Press release.

Report of Office of Special Counsel addressed to President Obama:

As explained in the accompanying report, OSC concluded that Secretary Castro
violated the Hatch Act by advocating for and against Presidential candidates while giving
a media interview in his official capacity on April 4, 2016. Secretary Castro’s statements
during the interview impermissibly mixed his personal political views with official
agency business despite his efforts to clarify that some answers were being given in his
personal capacity.

The Hatch Act prohibits federal employees from using their official authority or
influence to affect the outcome of an election. While federal employees are permitted to
make partisan remarks when speaking in their personal capacity, the Hatch Act restricts
employees from doing so when using an official title or when speaking about agency
business.

As the upcoming Presidential election approaches, this report offers an
opportunity to deter violations by reminding federal employees of the Hatch Act’s
restrictions.

Castro's response to Special Counsel Lerner:

Dear Ms. Lerner:
I have received your analysis of an interview I provided to Yahoo! News last April.
In responding to a journalist's question about the 2016 election, I offered my
opinion to the interviewer after making it clear that I was articulating my personal
view and not an official position. At the time, I believed that this disclaimer was
what was required by the Hatch Act. However, your analysis provides that it was
not sufficient.
Thank you for bringing this matter to my attention. When an error is made - even
an inadvertent one - the error should be acknowledged. Although it was not my
intent, I made one here.
I appreciate your staffs diligence and professionalism in assessing the content of the
interview and the circumstances surrounding it, as well as the clarifications of law
that your team provided. As a result, I tasked the Executive Team with enhancing
training of the Act's requirements and adjusting how these types of journalistic
interviews are handled in the future.

Regards,

Julian Castro

UPDATE: In the offending video , Katie Couric asks Castro about his support of Clinton right at about minute 7:00. (I haven't yet found an embed code)

So, is this the end of that? Anybody see any reference to who reported him?

Oh, and, anyone know how many of the Veep contenders have been investigated recently?

A bit of history on The Hatch Act:

Under the leadership of Thomas Jefferson, the nation’s third president, heads of the executive departments issued an order which stated that while it is “the right of any officer (federal employee) to give his vote at elections as a qualified citizen... it is expected that he will not attempt to influence the votes of others nor take part in the business of electioneering, that being deemed Columbia and certain employees of state and local governments." (pdf)

In passing the Hatch Act, Congress affirmed that partisan activity government employees must be limited for public institutions to function fairly and effectively.