Have you controlled the 75-100k income results for age? I could imagine it including quite a number of people that are already retired or close to retirement.
In this case, you could have a lower educational attainment level than for younger people in the same professional group. Let's hypothetically take the hospitality sector (a sector that is typically assumed to lean Democrat) - older middle-management staff might be high-school only, younger ones would have undergone some college education.
Asides, as I have noted in
this thread (scroll to the end), the white 45-64 age cohort, and especially the 60-64 subgroup (2008 age classes) seems to display an above-average Democrat lean and was also more resilient to the 2012 white pro-Republican swing. This may on one hand relate to political socialisation ("JFK generation"). In addition, this age group may have preferred Democrats due to specific issues (2008: Financial crisis and its effect on pension funds, 2012: Obamacare / Medicare).
