The reasons that seemed to get a quick dismissal were people who had been victims of crime in the past, people who had family members who were police officers, lawyers, judges. People who held not uncommon beliefs that would bias them (e.g. "I believe that people of color can't get a fair trial"), but not always if the juror said they could make a decision without that bias clouding their judgement.

The ones that picked extreme beliefs just got more questioning until they either admitted their belief wouldn't cloud their judge or that their belief wasn't so extreme as to bias them.

I mean, the judge and lawyers question people for a living, so they know exactly how to drill down and force an answer out.