All TED events are managed by a committee, including one that oversees the speaker selection. Often, committee members have some ideas on who they would like to attend and speak at the event. However, all of the events also accept nominations for speakers as well, especially those who fit their specific theme.
After giving a few hundred speeches in the past year, I've been struck by the variety of ways that different people introduce the same speaker. Some introductions energize me and seem to leave the audience excited to hear from me. Other introductions inadvertently make it more difficult to deliver a successful speech.