Why do trainers sometimes lower the pitch of their voice?

When working with horses, you might sometimes hear the trainer intentionally lower the pitch of hers or her voice. Why do we do this?

It's the "mommy" voice. A mare lowers the pitch of her vocalizations when communicating with her foal. So, a vocalization at a lower pitch, especially one obviously lower than the person's "normal" voice can have a greater impact on a horse and elicit a better response.