Hi Wes, I too had relatives that moved from NE TX to Hill County after the war. This was after moving to TX from their devastated area of MS. The Handbook of TX Online at http://www.tshaonline.org/handbook/online/
does not give up many clues as to a specific reason to migrate there, but the bibliographies quoted from might:

BIBLIOGRAPHY: Ellis Bailey, A History of Hill County, Texas, 1838-1965 (Waco: Texian Press, 1966). J. C. Daniel, A History of the Baptists of Hill County, Texas (Waco: Hill-Kellner-Frost, 1907). Hill County Historical Commission, A History of Hill County, Texas, 1853-1980 (Waco: Texian, 1980). A. Y. Kirkpatrick, The Early Settlers' Life in Texas, and the Organization of Hill County (Waco: Texian Press, 1963). Fay M. Little, History of Hillsboro Junior College (M.A. thesis, Baylor University, 1964). A Memorial and Biographical History of Johnson and Hill Counties (Chicago: Lewis, 1892). James Verdo Reese, A History of Hill County, Texas, to 1873 (M.A. thesis, University of Texas, 1961).

I'd like to know if you learn more.
Fran Bolton