Union past for sure would be the biggest reason, especially in the Northeast. Now that the cameras are rolled out, they are going to be especially careful to keep the unions away. They have been having anti-union meetings lately as well.
That has to be it, because having no tickets and no accidents on your record with a CDL is like having the golden goose. I know quite a few guys with tickets on their record that got hired here no problem. Even I had a speeding ticket on my record when I first hired on.