I think it has a lot to do with having a competitive nature and getting the point across to the players. They can't do it themselves on the battle field so they have to really push hard what they teach and sometimes that comes across as yelling. I don't think they try to be mean but come across that way especially when the players don't listen or do something wrong, then it gets worse. They play good less yelling play lousy more yelling.
"Let's do it again" David Glass