my gelding does that in a morning when he sees me walking down to turn them out.
and also on an evening when they are due to be put in and he sees me with the feed buckets.
when i first got my mare, she used to do it, she did it that bad she had to have front shoes on when she was 2 years old, she is 18 now, and she hardly does it now!
I don't think it is an unusual habit at all. It is their way of showing impatience.
My horse does it when I stand chatting to people too long and keep him waiting or when I'm in the field with my other horse when he is doing it for attention seeking.