Sometimes when people are nervous, they tend to speak too fast. It is an understandable condition, and may or may not have anything to do with how prepared the speaker is. Some nervousness is a good thing, because it keeps you from becoming complacent. When I have been very comfortable prior to giving a talk, I do not do as well as when I am somewhat nervous prior to speaking.
Another reason I believe speakers talk too fast, is to keep their audience from interrupting and asking a question they may not be able to answer. I’ve heard a consultant with an international consulting company speak several times, and she talks so fast that my colleagues at one law firm referred to her as “rapid fire Sue” (not her real name). When she was interrupted, which was not an easy task, she seemed to flounder around for answers.
A third reason speakers “jabber,” according Tom Antion of the Great Public Speaking blog, is an effort to keep the audience interested. As Tom tells us, good speakers use pauses to make their speech more effective. Take a look at his post entitled “Public Speaking Tip: Pauses” for his suggestions regarding pauses of various types.
Speaking too rapidly is never effective, IMHO, and should be avoided at all costs. Not only can pauses (both planned and spontaneous) make your delivery more compelling, but they can help overcome the tendency to speak too quickly.