Wouldn't it be the reverse? The word unreasonable is often used as a synonym for volatile, unpredictable, even dangerous. That's because "reason" is viewed as highly predictable. Two people who rationally reason from the same set of known facts would be expected to arrive at similar conclusions.

I think what Ilya is trying to get at here is more like: someone very smart can seem "unpredictable" to someone who is not smart, because the latter can't easily reason at the same speed or quality as the former. It's not that reason itself is unpredictable, it's that if you can reason quickly enough you might reach conclusions nobody saw coming in advance, even if they make sense.