Once USC and UCLA left, you know Oregon and Washington were immediately on the phone with the Big Ten. From the very beginning they must have been given adequate level of confidence that they would eventually get invited to join. They now had no incentive to lock themselves into a long term grant of rights.

Now, you are left with 8 schools. Half of those have no real value as an athletics revenue generator (WSU, OSU, Cal, Stanford). The other half didn't have as deep of roots with the conference, so they put self above conference and actively worked on joining the Big 12.