Which was the county which every elected Democrat did so at the same time - Shackelford or Throckmorton?
Throckmorton in 2012.
In Texas, primaries are conducted by the political parties, and in particular the county political parties. If there is no county chair for a party, there will be no primary. You can run as an independent, but that requires a bit of effort, while filing in the primary just requires the filing fee.
There is little political competition for county offices. So everyone may just go along and be a "Democrat". No one is going to vote against Bobby who running for sheriff or Mary Sue for county clerk because they're a "Democrat". The only detriment is that you can't vote for statewide offices, and perhaps for legislature.
Even if there were a Republican party and primary, voters might pick the Democratic primary because that is where the local officials are chosen.
All the news accounts for Throckmorton County were in early May, a few weeks before the delayed primary, and were associated with restoration of the county courthouse, and include some state dignitaries such as the Ag Commissioner and the state party chair.
The candidates would have had to file in March, and more likely in December 2011. So they may have just taken advantage of the county event to get reporters from Abilene to notice. There was also a bunch in Coke County who switched, and that was reported in the San Angelo media in April.