Roughly in the mid 2000s it got very common for people in the trading business to get on chatrooms. Bloomberg is probably the best known, but somehow Yahoo Chat also has a piece in the commodity space.
The chats go on constantly, and I would guess people just didn't think anyone would look through the records. Much like with squawk boxes, who would ever imagine someone listening to the recordings?
Similar cases have happened in FX and LIBOR, and it's quite clear what the incentives are. You might think that some banks are long and some short, thus cancelling out the market effect, but that's not the case. Often the trade that's popular is the same for everyone, so the banks are on one side and the customers on the other. So if you have a small group of banks that in a chatroom, it's quite tempting to collude.
You also have to think about how market making works. It's at heart an information business. If you know customers are going to be buying, you want to get inventory first and sell it to them. It's nice to be the first guy to do that but realistically you won't know what's up all the time. Makes good sense to have friends at other banks.