Excessive personal demands. No public speaker is perfect. If we demand perfection of ourselves we will usually be disappointed. For this reason, the slightest stumble or hesitation may send us into a flurry of nerves and anxiety. If we don't make a big deal of it but move on without making faces or frowning then we give the audience a chance to forget about it.
Recovery is a key.
Be simple and direct, without all the self-blaming some speakers fall into. If we are poorly prepared, if the speech is disorganized, poorly documented, or lacking in interest, we have real cause for fear. This is probably the biggest reason for rational fear. If we have not practiced out loud, only reading in our heads from a manuscript, we have no idea what we will sound like when we do speak in public. Suddenly sentences sound awkward and words leap out that we're not sure how to pronounce.