All wiggins aside, an opinion, which one may express, is that pros are by their nature, competitive, sometimes, very much so. Noobs by contrast, may be seen as hesitant or inexperienced.
Noob Pros though by contrast are probably not very hesitant at all, even if inexperienced. They dive in, take their licks and get competent fairly quickly, or drop out, though they are less "emotional" about it, than perhaps more retiring noobs.
As to why they "Team Up" on noobs?
That can be answered, both easily, and to a greater depth, not quite as easily, they are combative, confrontational and are expressing simply the side of themselves verbally, that intuitively gets them to the top of whatever endevor they are persuing. Business and politics are abound with it, so is pro sports, so is gaming. 
If we have a bad mannered individual dumpng on us in a game, calling us idiots, or @#$% off, or whatever, it is because such behaviour in the past has led them to a greater degree of success or control in what they are doing.
Also its arrogance, being good at something, may imply to someone who is confrontational in nature, that they are "better" than, or "outdoing" whoever it is they are harranguing.
Among themselves, this is quite acceptable behaviour, and how they establish a "pecking order" of who is the best, who is second best and so on. Amongst themselves this is simply the way the pack behaves and it is all taken in stride and there may or may not be any personal meaness involved, simply out-manuevering of one individual against another in the same pack.
With noobs, however, we intellectualize, we see personal motivations in behaviours that simply are projections of how "we" deal with things, "how" we perceive things. The Pro's teaming up, or dumping on, or cussing out, or other "confrontational" behaviours are seen by the noobs as "personal".
Usually this is the farthest thing from the Pro's mind. If they are strutting and cussing, it is usually not for our benefit, but for the benefit of the "pack" they are running with. To appear bigger, badder, and better than the noob who just can't seem to "get it". It's not us that they are trying to impress, its the "pack" that see's them as "boy, did you see what he said to that guy?" kind of thing.
A lot of this is pyschological, it's also very much sociological, in that it is primarily concerned with pack models and pack behaviour and pack rules.
As a noob, the best thing we can do is to remember that the "pack" we run with has a different set of rules and behaviours and "talk" that we use to identify ourselves with, and to avoid as much as possible, or mix with the "Pro's" but understand why they do what they do. Either accepting the "confrontation" as a part of the experience and trying harder to get better, or simply not interacting with them.
By reverse analysis, it can be noted that the Pro's feel they are "putting" up with all the incompetence and inexperience for nothing, that it is a "waste" of their time. Such attitudes and behaviours often lead to success in any given task, business or politics or sports or gaming, but in personal relations often suffers. Marriages fail due to infidelity, or affairs, or at least in music, one rap artist shoots another for bad mouthing him.
So, there we have at least, some reasons why Pro's do what they do, and why noobs react the way they do. But remember that human psychology isn't defined in half a dozen paragraphs and that people are all kinds of ways, so while "pack" behaviour pertains to some of us, to others it may mean nothing at all, their motivations may be personal and vengful and plain mean simply because of the psychological "edge" it gives them over whoever they are confronting. 
Happy gaming and modding and psychologizing!!! 
Oh, just a thought, sometimes, online multi-player is more "friendly" with people you know, instead of the "anonymous" faceless internet handle that we see in a game lobby somewhere. So if you like multi-player and hate getting dumped on, find some friends, either close to home, or people on the forums that you know that seem to have the same disposition you do, and then logon and have a great game. 
Take care and keep playing!!
-Teal