Probably some abuse of a bug ? The only time TQ rollbacked the French servers was due to the chicken bug (a special quest for Thanksgiving I think, where in exchange of a chicken, you would get some EXP, but for some reason the 5 times per day was not respected at all). The other time big bug (I think it was related to reborn gems, but it's before 2nd RB, etc, so a long time ago), the GM just asked for the cheated items and most gave them back. Others were banned.
Brief, maybe a big bug abuse that the only way to fix is to rollback. Or an issue with MySQL/TQ's servers, which I doubt.