Basically there are three kinds of screamers:

1) Persons who refuse to accept that things won't always work as they should and get terribly upset when they discover life is less predictable than they think. They will get defensive and start screaming because of their insecurity.

2) Persons who consider themselves important, usually by virtue of combining a job in middle management or finance with a mega-ultra-super-duper UeberAllianz Titanium FF card. The world has to stop for them and the airline has to arrange their transportation to the final destination using the personal plane of the Sultan of Brunei. They scream either because it makes them feel superior to the GA ("I AM ONE OF YOUR BEST CUSTOMERS, TAKE CARE OF ME NOW!") or because they think the other person is far too stupid to understand their instructions.

3) Persons who scream deliberately as a means to their end of getting their wishes attended to. Bullying works more often than it appears to at first glance. If you think about it (and I mean no offence to any GAs), most (non-supervisor) GAs will tend to be genuinely nice people who are unable/unwilling to get a job that would require them to be confrontational and bargain really hard. Plus it is likely that they had been stressed prior to you starting making demands from them therefore one could, with the right use of tones, words, and body language, intimidate the GA and get what they want.

No. 3 is a little art that's quite difficult to perfect, but I do know one or two people who are good at this. I wouldn't do that type of screaming when faced with a GA, but I (knowing that this will not make me any more popular with fellow FTers) will admit to doing it sometimes when faced with a particularly serious and tricky situation for which I am not responsible and a scapegoat has to be found (as opposed to accepting the situation as it is or putting the blame on somebody who cannot assist in resolving it).