Basically when the computer gets to the end of a run and there is a given day or set of days that has too many trips for the reserve pool to cover, the computer will go back and assign trips in inverse seniority order to us to get down to an acceptable amount of uncovered (open time) trips on a given day or set of days.
Really upsets the guys in the 50-60% range in December because they are always unstacked and never see it coming.
Basically unless you are above the 50% you can have your preferences overridden so that the company meets required manning levels.
It happened the first weekend of Aug too. (Last weekend before all of the kiddos went back to school) Lots of mid to junior line holders were unstacked.