Interesting, but not what happened. The trips were all assigned to pilots already. A scheduling enhancement tool (controlled by the company) was turned on for the pilots as it does every month. Usually the company tightens manning tolerances (for lack of a better description) to about 99%, requiring pilots to trade one trip for another if they needed to get a particular day off after their initial schedule has been assigned, with 'undesirable' trips (like those over holidays) usually not tradable at all. This particular month, the tolerances were set at 0% for an as yet to be determined reason, which allowed pilots to outright drop any trip they had been assigned, without pay (unless they used a vacation day in the drop process). Thus, all the holiday trips got dropped. Some pilots even decided to take the whole month off.