It didn't seem like nerves. It appeared that he was thinking through some of these things on stage in more depth than he had previously done so the ums provided the time to collect the thoughts in a coherent manner.
that reminds me - I can't find the link now, but - someone had counted the vocalized pauses of college professors and averaged them by subject. They found that professors of math and hard sciences used the fewest, and that professors of philosophy used the most. Presumably, because, when there's less of a definitive answer to a question, you must work harder to come up with words to describe it.
