I heard there are several reasons for dropping to 91 octane. A couple are technical reasons from the sister of the hairstylist of the accountant of a guy who works at the Tosco refinery. The one I remember best is that they can lower the price of premium by lowering the octane.
Guys I know who run dataloggers note that they aren't seeing more knock sensor activity with the lower octane gas. Maybe it's a matter of time until the the remaining 92 octane gas is completely out of the supply system.
BTW, it started in the Bay Area in July with full implementation by August 1st.