Can you imagine the chaos if Matt was specific? "Today we are doing an update that increases the value of the xyz type of link." Everyone here would be buying xyz types of links. Or if he said "This week xyz types of domains are going to lose value". How many people would be bailing out of xyz domains?
My best guess is that this large update has to do with the mud on the face that G and Matt received with the payday loans by Matt Cutts fiasco that happened recently. Gotta love the guys who did it. Payback is hell. :)