In Travis turnout might partially explain it. The raw vote total in Travis was about 3.2% lower in 2012 than it was in 2008, meanwhile it was down 1.1% statewide. During the same time period the population growth was more than double in Travis 11.7% compared to the state as a whole 5.4%.
The 3.4% raw vote drop off was larger than the country as a whole 1.7%, so Travis had a larger drop off than both the nation as a whole and Texas as a whole, despite the fact the county grew at a much larger clip than both. Chances are the bulk of that dropoff was younger voters.
The same is true in Fulton, the drop off was close in line to the national drop off, 1.8%, but higher than the state drop off .6%. This is despite the fact that Fulton grew by 10% in population between 2008 and 2012 far outpacing the nation as a whole or the state 3.6%