If everyone’s preferences leave too many trips over a certain section of the month they “unstack” a certain percentage of pilots and force those trips on those pilots during those days. It all depends on the rules and language when it can be done and how much of the pilot group is affected. Could be done every month but mostly on holiday months. Good rules will have it controlling only certain days like 2 days before Christmas to 2 days after and have a separate bid sheet if you happen to be in that percentage. It’s all in the rules and oversight, typically it’s the bottom 35-50% or so that are affected.