This would be the Virgin effect, except there's no Virgin in Denver.
For many months Southwest would match Virgin's sales at SFO and add $10 for OAK. A few weeks ago, Southwest stopped matching at OAK and kept matching only at SFO.
I think this is the United and Frontier effect. Neither one of them go to OAK, but they both fly to SFO and SJC.
I think the Virgin effect at OAK has come and gone. It might be back in the summer, if demand stays low. I've actually noticed SJC has better fares than OAK. It used to be the other way around.