There are many factors at play in determining the causes of vineyard conversion, many of which are speculative. One of the most glaringly obvious is the economical component: according to a study in the North Bay Business Journal, the combined economic impact of the wine business on the Sonoma County economy was $13.4 billion in 2012. Furthermore, the wine industry, directly and indirectly, provided 54,297 full-time-equivalent jobs in Sonoma County paying $3.2 billion in wages (NBBJStaff, 2014). There's tremendous economic incentive for Sonoma County and others in California to allow major wine conglomerates to plant grapes on as much land as possible.
But dig a little deeper, and you'll find--as Alastair Bland found in his article in The Bohemian--that, “Some environmentalists say that legal lenience toward the Sonoma County wine industry can be traced back to the 1970s, when the threat of suburban sprawl spilling off the Highway 101 corridor was staunched by amendments to county code that gave agricultural lands legal precedence in the fight to survive. Today, that agricultural land has become mostly vineyard land" (Bland, 2011). The amendments designed to keep food or fiber farms afloat now directly benefit vineyards.
Climate change may also be a factor. According to Stephen Fuller-Rowell, a cofounder of the Sonoma County Water Coalition, "global warming will begin driving grape growers into increasingly higher elevations as they chase the cool climes favored by Pinot Noir, a trend already at work in Oregon. In Sonoma County, this could mean increased pressure on hillside and ridge-top regions lacking in water and susceptible to erosion, and lax county laws could facilitate this uphill migration." His colleague Jane Nielson says that fines against lawmakers aren't stiff enough either, allowing winemakers to simply absorb the hit, "calculate their benefits into the fines they'll be paying," and proceed as planned (Bland, 2011).
But the worst of these loopholes is "piecemealing," wherein large projects are divided into a series of less-imposing smaller ones. By nudging these bits and pieces through the review process, you can effectively push through the entire package. "The Cornell Winery," Laura Waldbaum says, "is a prime example." "First, Cornell ventured in some illegal logging, acquiring a permit later in time. Then he got the go-ahead for a vineyard. Now, he is pushing through his winery. Meanwhile, he has bought up several surrounding properties which Waldbaum believes will eventually be turned into vineyards" (Bland, 2011).