without knowing what city pair this is its hard to speculate....

for example there could be a convention that week going from say Thursday thru Sat AM, Many of the people are traveling out that afternoon or on the flip side there is a large convention going on in this destination starting on sunday and from this city many travel to this convention so many seats are already tentatively booked as part of the convention attendance similar to hotels holding rooms for conventions but releasing them when under 60 days.

This also could coincide with a college basketball teams traveling. outside of normal travel you have this team and staff and local press that travel with the team at 30+ extra seats sold