Peter Mead at the Biblical Preaching site refers to something that I suspect is very common. That at the most inopportune time a distraction happens. It may be a baby crying. It may be someone walking out in a distracting way. It may even be someone showing up late.
It could be any number of things, but it is often right when you are hitting a grove, right when someone needs to hear what is being spoken, right when God is about to say something important. Mead notes several possibilities of what is going on at that time. Personally, I think what we are seeing is the battle between good and evil playing out. The enemy of our souls was looking over our shoulders as we prepared the message and set his plans in motion of how to counter the message. We are in a warfare with the legion of darkness and the preacher cannot forget it.
But the good news is that Satan can attack but greater is he that is in you than he that is in the world. We have nothing to fear for God is with us as we preach the message of truth. I remember one preacher coming upon a particularly poignant part of the sermon and the lights just went out. The preacher then used the lights going out as an illustration of in the message.