The mystery was the buzz of Wine Country: why would the president of one of the region's fastest-growing wineries, 585 Wine Partners, abruptly leave the Sonoma company he co-founded?

The answer came Thursday, when a company spokesman revealed the internal coup had been orchestrated by silent investor Fred Franzia, the pugnacious vintner whose Ceres-based Bronco Wine Company makes inexpensive brands like Napa Ridge and the $1.99 Charles Shaw.

A Franzia family partnership called Barrel 10 Quarter Circle "took over" at 585 late last month, ousting president and co-founder Dan Leese and his wife Kathy, who handled public relations, Bronco spokesman Harvey Posert confirmed.

"Fred said he had been a partner in this thing, and the Leeses apparently were let go or left. I don't think it was of their own volition," Posert said.

He declined to offer a motive for the move.

Dan Leese, an industry veteran who helped grow the company into one of 30 largest wineries in the nation in under five years, stepped down Feb. 1 without explanation. He declined comment again Thursday.

Franzia installed longtime Bronco executives Dan Leonard as manager of the business and Jim Carter as director of operations, Posert said.

The shake-up surprised many in the North Coast wine industry, where Franzia's investment in the company was a well-kept secret.

People familiar with the situation say the move was probably triggered by a combination of slower sales of 585's wine brands, Franzia's zeal for cost-cutting, and his desire to be in control.

"That's Fred. He's big. He's bad and he's going to throw his weight around," said one industry source who spoke on condition of anonymity.

The Leeses launched 585 Wine Partners in 2005 when they bought a controlling interest in Red Truck Wines from Sonoma's Cline Cellars. The company later added brands including Steelhead Wines, a partnership with Dry Creek's Quivira Vineyards, Picket Fence Vineyards from the Russian River Valley, and imported Bivio Italia.

Franzia, who has famously remarked that no wine should cost more than $10, is probably interested in increasing production of Red Truck, making it with grapes from his 45,000 acres of vineyards, and lowering its price, sources said.

The Red Truck wines, which make up the vast majority of the company's 500,000 case production, currently sell for between $9 and $12. The others in the group sell for between $15 and $40.

Leese has said he remains "an ongoing partner" in these other brands and "will continue to play a significant role in their development."