Best Buy Co. Chief Executive Brian Dunn resigned abruptly Tuesday amid what the company described as a probe into his "personal conduct," adding to the woes of the slumping consumer-electronics retailer, whose big-box business model is crumbling.

"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," the company said in a statement....