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The doctrinal differences between Regular and Separate Baptists were slight. The Regulars were indeed Calvinistic and largely descended from British and Welsh Baptists. The Separates were also Calvinistic but descended originally from the Congregationalist churches of New England; during and after the Great Awakening, some of the New Lights from Congregationalism left the fold (or Separated, hence their name) and became Baptists.
The Regulars and Separates in the South, for the most part, became undistinguishable by the beginning of the 19th century, sometimes going under the label of United Baptists.

I have some references for your question that I can assemble.

 

Tom Nettles, a leader of the Founders movement (which seeks to re-emphasize the doctrines of grace, or however you wish to describe it) lays much of the blame at the feet of E.Y Mullins at Southern Baptist Theological Seminary.

http://www.founders.org/journal/fj19/article1.html

(BTW, Al Mohler deals with Mullins at length, and in a more nuanced manner, at
http://www.albertmohler.com/2009/07/16/e-y-mullins-the-axioms-of-religion/.

Jim Maples has reviewed the literature in his doctoral thesis, "The Point of the Southern Baptists' Departure from the Doctrines of Calvinism.," which can be found at http://www.rts.edu/Site/Virtual/Resources/Student_Theses/Maples-Southern_Baptist_Departure.pdf. Worth the read.