Yeah that's the thing that always bugged me as well, as far as i can tell there are two main motivations.
1. They suck at the game and like to see a big score next to their name even if thy have to cheat to get it. This is obviously based in some serious self esteem issues, basically they have no life, no friends etc. It's a bit of a stereotype but it's still true.
2. They do it specifically to annoy people, classic troll behaviour. The motivations of a troll are many but they're all still based in some very negative ideas the person has about themselves. Whether it's low self esteem, the need for attention (good or bad) or just because they are bored, once again they have no life.
There may also be a power element involved, thy are powerless in real life and so getting someone to react to them online shows they can control another person.
So yeah that's how i see it, basically these people are to be pitied. They may win a match but they lose in life.
Source - 1 years worth of psychology.