Also I can think of plenty of other causes that are equally or more plausible than the ones cited by the authors:
* MSNBC launched in 1996.
* 1996 presidential election, hotly contested because Democrats wanted to take back the House from the GOP (did not succeed) and Second Amendment advocates were alarmed because of 1993 and 1995 anti-gun legislation. There was the 1996 FBI white house files controversy, Clinton signing the Defense of Marriage Act in 1996, etc.
* Drudge Report launched in 1996 (followed by Lewinsky scandal in 1998).
Though I think greater access to alternative media via the Internet, no matter what your political persuasion, is the most likely cause of greater polarization. You no longer had only ABC|NBC|CBS and your local newspaper and radio station. Republicans now had Drudge, Democrats had MSNBC.com, socialists had wsws.org, libertarians had Cato.org or Reason.com, etc.