Most popular reason is probably revenge. Doesn't really matter the reason. Simply fill out the report form and send it in. When enough reports are made, action will be taken by the site.
player who abandon game.
Such action is indeed really annoying and idiotic. It seems that some just can't accept losing game and want mess with other player instead just resigning or playing until end. I just report them and yet ignore them. I don't know it prevents them play with me but at least they can't really communicate with them. I don't really bother with such people who can't play with good sportsman way.
I come across very rude player who just abandon game leave you waiting. Why cannot they just resign when it is obvious they have no intention of continuing. ? In my 30 min chess. i have waiting for the next move from a missing player for over 20 mins until the time 30 mins is up and the game is abandoned. So frustrating