We're talking about assembly of the Mac Pro not semiconductors. Presumably Apple chose Texas back in 2013 because they gave the biggest tax break, just like every other company.
National Semiconductor has a fab up in Arlington Texas.
I'm not sure if Texas Instruments still has their fabs open or not, but they had several at one point too. Sure there are more that I don't know about.
Though chipdesign and not fab, also worth remembering Apple acquired Austin-based Intrinsity in 2010 (https://en.wikipedia.org/wiki/Intrinsity)