This story has been updated.

GOV. MAURA HEALEY said on Monday that Transportation Secretary Gina Fiandaca is stepping down after seven months on the job, with neither official offering an explanation for the abrupt departure.

Speculation abounded over the weekend that Fiandaca was on her way out, but no one in the administration would confirm it. Theories about why Fiandaca was departing varied, from concerns about her laid-back management style, some of the hires she made, and her overly aggressive pursuit of an upcoming vacancy in the top spot at the Massachusetts Port Authority, even though she serves on that agency’s board.

Monica Tibbits-Nutt, the No. 2 person in the Department of Transportation, will serve as acting secretary once Fiandaca steps down on September 11. Healey administration officials said Fiandaca would remain in an advisory position through the end of the year to assure a smooth transition.

Tibbits-Nutt is very familiar with state transportation issues. She has previously served on the MassDOT board and was the vice chair of the Fiscal and Management Control Board, the predecessor to the current MBTA board. Prior to assuming her current deputy secretary position, she was the executive director of the 128 Business Council, which oversees privately-funded bus routes serving businesses along Route 128.

In her time on the MBTA board, Tibbits-Nutt was a fierce advocate for reduced fares for low-income riders. Sources say Tibbits-Nutt will serve in an acting capacity as secretary but the job is hers to lose.

A number of actions by Fiandaca raised eyebrows in the governor’s office, which of late had taken over many of the routine functions of the MassDOT agency. For example, the recent public announcement of a series of federal grant applications was run out of the governor’s office and not out of the Department of Transportation.

Earlier this year, MassDOT hired a firm, Teneo Strategy LLC, to help the agency plan a response to a federal audit of MBTA safety procedures. Teneo employs William Bratton, the former Boston police commissioner and Fiandaca’s former brother-in-law. Fiandaca and Bratton met privately before the agency awarded a $900,000 no-bid contract to Teneo.

Fiandaca also hired members of the transportation staff at Boston City Hall, where she previously worked. Even after the governor’s office raised concerns about the hires, Fiandaca hired  more people from City Hall. Two of them were assigned to an Office of Possibilities.

There were also concerns that Fiandaca, who comes across as shy and soft-spoken in public, was marginalizing Tibbits-Nutt and micromanaging some of the work and hires of Phillip Eng, the general manager of the MBTA.