I think the shift in political affiliation had something to do with Regan and the de-regulation of many different business sectors by the republicans in the 80's and 90's.

It became 'easier' to make money and with lower taxes those people got to keep more of it (at the expense of other Americans). So as these boomers started getting out of college in the late 70's and 80's they could now become stock brokers, bankers, real estate brokers, etc instead of a 9 - 5 factory worker. And those were the industries that were getting a lot of lower regulations from republicans so why not vote for their best interest and leave the left wing ideals to someone else.