Interesting what people are saying...
When a horse lowers his/her head, its usually a sign of being submissive. A horse who isn't submissive would have their head high in the air and a lot of other factors but I won't get into that.
But my filly does that, only lunging though.
Usually it just means they're calm, submissive, relaxed, etc. Its a good thing.
Here is my girl:
(If you want to skip the intro just go to 0:30 in the video)
Also kinda helps that Faith is bred to be more of a laid-back, level headed horse.