I suspect that Fran's thesis as to rich farmland is the primary reason. While there had been settlement in Hill County prior to 1860, it was close to the fontier, and subejct to sporadic Indian raids. Taking into consideration the absolute desolation of many areas of the South as a result of the war, both from combat and wilful depredations, many Southerners moved west to seek to restore their fortunes. Whether they settled on what had been the frontier in areas such as Hill County, or whether the pressure of their settlement further east encourgaed others to move further west, the undeniable result was a significant migration into the lands of central Texas.