My horse likes to tell other horses off when they step slightly in front of him, alongside is fine, but try to take the lead and he is very cross, ears flat back and pretending to bite. He only does it on the way home though, I think its because secretly he is a bit scared on the way out and really wouldn't mind someone else leading ;D
Think of it in a herd situation - the leader insists on LEADING, and disciplines those who try to take the lead. It isn't a secure confident leader though, rather one who has to constantly reinforce his position. Is he boss in the field - and more importantly does he see you as HIS leader?