Case in point, my boss at the moment does something I've noticed quite often in stupid people. As soon as the train of thought requires more than about 10 or 15 seconds of quiet thinking and analysis, he immediately changes the subject. He either changes the subject to something that (to my mind) is completely unrelated, or he makes some vague self-affirming statement.
On the one hand, this is annoying as fuck. On the other hand, if I look at it objectively, his brain is just wired that way. He doesn't have the capacity to break down an argument or situation to its atomic level, so his brain compensates by changing the subject and continuing the analysis, albeit on another topic.
It's really just another manifestation of the idea that our brains are constantly analysing things, but it just so happens that this manifestation is incredibly frustrating to anybody who enjoys getting to the bottom of and solving problems.