...because if something goes wrong they (and Utah) could be left out.
For example, if the PAC10 media deal is low (as rumored to be $20 mil) and if the Big10 decides it is not interested in Oregon and Washington for the next few years, those two schools may decide to join the Big12.
I think that is actually the worst case scenario for the PAC10: losing just Oregon and Washington but still "together".