This makes a lot of sense once you consider the VirginAlaska war for California.
Its a 3 way horse race with United owning San Francisco. This ownership allows them to serve such deserving routes such as STS and MRY to SFO, which only fly the 3 days that SFO has clear skies
Meanwhile VirginAlaska has upped the stakes with small planes flying plases like STS to SNA and LAX to Mammoth.
Meanwhile WN is the current leader for people flying NorCal to SoCal, but the other two above fly to a lot more airports so WN needs to catch up