Hi Zeus
Apologies for this, the RuneScape FriendsChats are designed so that if the FriendsChat is full and a ranked player then joins, it will automatically remove another player from the FriendsChat. It's annoying for everyone involved as nobody wants anyone to be removed for no reason. It's a shame they couldn't have fixed this in the latest Chat Updates.
This could very well have been the reason. The only other reason I could think is that it was simply a mis-click when another player was being kicked from the chat. Either way, I hope you've had no more issues with this since you posted.
If you have, please send me a PM in-game or post here so we can investigate it further.
Joey P