You're Being Watched...
Here's an observation about human beings speaking in workshop settings: Some people will walk up to the front of the room and quite happily chat away to the group.
Other people, however, will walk up to the front of the room, start to talk, and immediately lock their eyes on you, the Facilitator, and only look at you for the whole of their presentation. Forgetting somehow the other 40 people in the room sitting right in front of them. This is a little bit perplexing for everyone except the speaker, who doesn't seem to notice.
What's interesting is that you can do something about this without saying a word or even (almost) anyone noticing. You simply walk slowly and quietly around the outside edge of the room, while the person is speaking, to the middle of the back of the group. You won't disrupt the flow at all. The speaker's eyes will follow you the whole way. And then you stop and stand there. Viola, the person is now talking directly to the middle of the group. Even if he is still looking directly at you, at least he is not talking sideways, craning his neck or otherwise looking away from the group.
Of course to use this trick, you need to notice that the person has locked onto you. So you need to be attentive to the speaker. Normally I find this happens when someone is not confident in the subject matter, or simply not comfortable speaking in front of a group, period. So a smiling nodding face (yours) is a comfort and a safe place to look. However, once you do notice, it is time to take action - start walking slowly and don't worry, you are being followed. Good for the speaker, good for the group, kind of flattering, and easily and gently corrected.