1. People either genuinely crazy or so woefully misinformed about how the world works, the bases for their decision making is so flawed they may as well be crazy.
2. People with a worldview which lead them to disagree with what you consider rationality even though they arrive at their positions through rational means.
3. See also birther
Obama vs. Alan Keyes is proof. Keyes was from out of state, so you can eliminate any established political base; both candidates were black, so you can factor out racism; and Keyes was plainly, obviously, completely crazy. Batshit crazy. Head-trauma crazy. But the twenty-seven percenters in Illinois voted for him. They put party identification, personal prejudice, whatever ahead of rational judgement. Hell, even like 5% of Democrats voted for him. That's crazy behaviour. I think you have to assume a 27% Crazification Factor in any population.