I'm guessing there are several reasons: a) not the color they wanted, b) thought twice about playing the opponent they selected, c) decided they didn't really want to play a game after all (e.g., scared, need to go), and d) they have no idea what chess etiquette is, having never played a single game of OTB chess for stakes.
Those are my guesses. I never abort a game that I have agreed to, so I can't add my personal reasons. I have aborted games as Black after waiting a minute or so for the other player to move and seeing no response.
We may never know.

Im using firefox and it has an ANNOYING ability to ask opponent for another game when i type. Im not sure if this is done by "enter" or "space". But sometimes when i write, the 'selection tab' just popps over to "Rematch" and I end up asking for another game. Ironicly enough, most of the times I type its to END a series of livegames, like "goodbye" or "later" and "thx for the games" etc. Sometimes it even happens multiple times in a row.

Why do people accept challenge for a Live chess game and then abort ? Whats all that about ?