There is also the introvert, a computer geek, accountant, etc. They don't play helpless, they are just quiet but confident.

I suppose they become aggresive because they don't have the patience to wait for the others or to explain....or no patience at all.Maybe they are in rush.
But those aggresive people are often so arrogant that i feel to punch them in the eyes.

On the last firm i worked for there were two of the workers who were very aggressive and were never wrong,and it got to a stage where people were having days off or were resigning because of the behaviour of these two people.I've never worked for a firm where morale and team spirit were so low.