you're right, they don't do it to be an asshole. they do, however, do it to feel like a badass, and they like feeling like a badass because they're an asshole.

if you start looking for it, or rather, start understanding that it's there, you will soon see an alarming number of people doing things solely because it makes them feel like a badass. it is 100% of the entire reason to live, in some people.

I suspect you are partially correct, they will roar out of fear to try to scare of the other guy, but after a successful take down, they'll also roar to show their dominance, in order to assert their position.