It's a speaking habit affected mostly by male speakers, in my experience. Typically, the arguments in favor of this stance in a presentation are that it's a "studied casual" look, something that exudes a relaxed, confident air. It also solves the problem many speakers anticipate of what they should do with their hands. But while this may work for models in the Sears catalog or on cool 1960s record albums, it doesn't work well for speakers.
I use a different argument against hands in pockets, typically citing Michael Erard's book Um. . .: Slips, Stumbles, and Verbal Blunders, and What They Mean. Few speakers realize that gestures are critical to them and their audiences. Gestures help the audience comprehend what you're saying, even when the gesture isn't precisely matched to what you're saying. But they also help the speaker's brain download to his mouth to produce the words he wants to say. If you immobilize your hands--by gripping the lectern, or putting them in your pockets--you are more likely to stumble in your speech, saying um, uh, or habadahhabadahhabadah when you really want to say something more eloquent.
I've found that a convincing argument, but my friend and former colleague Steve Tally just shared another, old-school (and pretty foolproof) method of curing this habit. His grandfather, the Rev. Sanford Ferguson, was pastor of the Tulip Church of God in Bloomfield, Indiana, and, it must be told, a big fan of hands-in-pockets while speaking in public. His wife, Glenda Ferguson, disagreed--a woman after my own heart.
Mrs. Ferguson, it turned out, had a better tool than most public speaking coaches do. She just sewed shut her husband's pockets. End of story.
Missing your needle and thread, or had this done to you? Read "What should I do with my hands when presenting?" for more useful alternatives.