When a lecturer is excited about what he is saying, he will often move away from the lectern. Increased body movement may also mean the material is important.
True; as people begin to lecture and /or discuss things they are passionate about, they tend to not only raise their voice and enlighten their facial expressions, but rather, they also cannot seem to stand still. Think of it as a preacher giving his sermon; once he gets going, he won't stay in one place.
