Are lecturers always nervous?

I like attending lectures of various sorts, but lately I have noticed a certain edginess by those giving the lectures. The specific type of lecture I am talking about is where someone is a researcher and has perhaps written a book on a topic and is giving a lecture to a general audience. The effect is particularly noticeable when the lecturer takes questions.

Could it be that researchers, although they may have given numerous lectures, are not usually comfortable talking in front of a lay audience? Perhaps reserachers tend to be more introverted. Whatever the cause, the effect is to detract from the appreciation of the lecture.