I'm deeply fascinated by the social effects of technology, but this is a particularly good one:
Before there was a long-tail internet full of self-reinforcing blogger and social communities; before there were 24-hour news cycles and Fox News; before there was cable even; there was something even simpler and more disruptive: air-conditioning. By making life in the South more palatable to Northerners, air-conditioning changed the country's demographic landscape, which in turn changed our political landscape. The migration to the South skewed towards older individuals and retirees, who tend to be more conservative. This simultaneously made the South more conservative and the North more liberal. As the Democratic Party became more liberal, the tensions with the conservative Dixiecrats grew until they gradually and then completely shifted to the Republican party. This led first to the liberalization of the Democracts and then to the hard-line conservatism of the Republicans.
Indeed, the first secure Republican seat outside of Appalachia was St. Petersburg, FL - a winter resort - and the second was Dallas, TX.
Credit for this theory and the much more detailed research and analysis that went into it goes to Nelson Polsby, one of the foremost experts on the US Congress.