It's funny that somebody brought up this subject, since a few days ago I was
just talking to a friend who's doing dressage training about exactly this
phenomenon. He said that after a period of dressage exercises (which require
full collection) the horse usually gets an opportunity to relax and stretch on
a long rein and is actually encouraged to do exactly what I noticed Scatris is
doing during longer workouts: stretching his neck all the way to the ground
and this way relaxing his back muscles.
In the beginning when he would do this I was also irritated. I thought my
horse is tired. But I also noticed that he will do this in situations where he
can't be tired, for example when we've been trotting on "easy" flat ground for
a while where I have to ask him gently but consistently to not start blasting
off. He has a big extended trot and I can imagine that it must feel good to
stretch out once every now and then. He will not even slow down noticeably
while doing this.
I think a tired "neck down" is different. You'll know that your horse is
exhausted even without his head reaching down. He'll be less balanced, over
all slowing down and probably a little tense in the back.