When you stand up to speak, you're risking a good deal.  You can fail to engage the crowd, you can make a fool of yourself, you can attempt too little or too much and miss the mark.  While the risk is almost always greater in your own mind than in reality, it is nonetheless a real risk.

Naturally enough, that's what's on most people's minds at the moment they begin to speak.  They're thinking to themselves, "Why did I agree to do this?  It could all go horribly wrong!  People are going to think I'm an idiot!" or something along those lines.

The result of that self-talk is a series of behaviors that increases the likelihood that precisely the feared result will occur.  People who fear failure in speaking are defensive, and that defensiveness shows up in a variety of ways, all bad.  They may pace nervously — the familiar 'happy feet' of some speakers.  They may clutch and un-clutch their hands in front of their stomachs.  They may cross their arms, hide their hands behind their backs, or keep their arms firmly fixed to their sides, only waving their forearms, in a characteristic gesture of many business speakers that I call the 'Penguin flap'. 

All of these gestures, and others besides, signal nervousness to the audience.  But more than that, they signal that the speaker is trying to protect himself.  The speaker, in fact, is shutting off part of herself from the audience. 

And that's the problem.  The whole point of presentations, from the audience's point of view, is to see the speaker whole, to gain insight into this person who has the authority to stand up and speak to a crowd.  If the audience sense that the person is holding back, its judgment is that the speaker is ultimately dishonest, and so can't be trusted.

That's not of course (usually) what the speaker intends, but that's the tough luck of public speaking.  The good news is that the audience desperately wants the speaker to succeed, and so is willing to grant the speaker a lot of nervous, self-protective leeway in the first few minutes before giving up entirely and writing the presentation off as a bad job. 

If you're speaking, then, try to begin right away avoiding self-protection.  Get over yourself and your nerves.  Be open to the audience.  If you can manage that, the crowd will carry you and give you back far more energy that you put out.  The irony of public speaking is that the biggest risk is not risking yourself.