I've head from lawyers that East Texas became the place to patent lawsuits mostly because the War on Drugs. It was much easier to get the courts' time for civil trials or motions before they were overloaded with constant criminal prosecution. East Texas happened to have a lower load at the beginning or some townships spotted an opportunity to bring in some more business by making themselves attractive to litigators. Either way, the reasoning things are filed in East Texas has more to do with it being the Silicon Valley of patent trolling than any kind of venue shopping.