It took just hours for speculation to heat up in Raleigh about the reasons why.
"This is politics and rumors run rampant," said GOP political strategist Carter Wrenn.
After 40 years in politics, Wrenn seemed qualified to weigh in on the persistent questions in political circles surrounding the Shanahan's resignation.
There are questions over whether Shanahan misused a DPS helicopter, or improperly arranged for a $10,000 raise for a female assistant in the State Highway Patrol office.
Wrenn suggested the speculation is politically motivated.
"The Democrats are getting the speculation in front of the facts and right now there's really no point in having a lot of speculation because, if there's nothing to it, it's just a resignation," said Wrenn.
Inside the Department of Public Safety, ABC11 was told Shanahan was not in and unavailable for comment.
However, the agency did release records showing Shanahan used the DPS helicopter just once during his tenure and it was for official state business. A spokesman said that the $10,000 raise was handled by human resources.
They did not say whether Shanahan played a role in the matter.
Governor Pat McCrory and Shanahan's resignation letter attribute the abrupt resignation to his responsibilities to outside business interests and a need to support his wife with her new promotion as a Navy admiral.
Time will tell if that quiets the Raleigh rumor mill.
"If there's something else that hasn't come out and it comes out, then it may make a political difference," said Wrenn, "but right now all you've got is a fella saying 'I tried it. It didn't work out. I'm moving on.'"
Shanahan's last day is Wednesday. He did not return calls from ABC11 Monday.