Brian Dunn speaks during a keynote speech on the opening day of the Mobile World Congress in Barcelona, Spain, Monday, Feb. 27, 2012. (Photo Credit Denis Doyle/Bloomberg/Getty Images)

After a 28-year career at Best Buy Co., Brian Dunn, who rose up from the sales floor to become CEO,  resigned abruptly on Tuesday as the company probed an undisclosed "personal conduct" matter.

The news of a personal probe sent shock waves through the business community.  Early on Tuesday, the company reported Dunn's decision to step down was a "mutual agreement." Only later did they disclose the probe.

"It seems like he's a pretty decent guy and a family guy, so I'm hopeful that he just added up something wrong on an expense report and it crossed the line and it's nothing worse than that,"  Wedbush Securities analyst Michael Pachter told Bloomberg TV.

In a statement late on Tuesday, the company said, "Certain issues were brought to the board's attention regarding Mr. Dunn's personal conduct, unrelated to the company's operations or financial controls, and an audit committee investigation was initiated. Prior to the completion of the investigation, Mr. Dunn chose to resign."

The company and Dunn declined to comment in detail on the reason behind the personal conduct probe.

"If they say it's not operational or financial, it's either inappropriate conduct or abuse of his authority or it's some kind of violation of company policy," Pachter told ABC News. "Those are the only three that it could be but I have no idea. Those are the kind of thing that would warrant an audit committee investigation and would prompt a resignation," he continued.

"They're so quiet about it that I tend to think it's embarrassing," said Pachter. "I tend to think it's something that has a second shoe. If it were sexual harassment, perhaps it came to attention because a claim was filed."

The investigation was first reported by The Star Tribune of Minneapolis.

Early on Tuesday, Dunn said in statement that at his departure the company was in position for a "strong future." Dunn, who never graduated from college, became the third CEO of the company in 2009 after joining the electronics retailer in the 80s.

In the same statement, the company wrote, "there were no disagreements between Mr. Dunn and the company on any matter relating to operations, financial controls, policies or procedures. There was mutual agreement that it was time for new leadership to address the challenges that face the company."

The departure of the company's CEO comes at a time when it's suffering as online competitors such as Amazon take a bite out of profit.

In March, Best Buy announced restructuring plans after weak quarterly results. The chain made plans to trim 50 of its 1,100 stores and lay off another 400 employees to cut costs.

"Best Buy's shares have lost more than half of their value since April 2006, when they were trading at $56.66 per share," according to the Associated Press. At press time, the company's stock was up 2.3% at $21.81 per share.