The big wheel slows as it spins up the smaller wheel (and the whole system slows in general due to friction, even when the two wheels reach equilibrium), but the governor's arms are sticky so even after the big wheel has slowed a bit, the arms are still extended a bit too much. If you give the wheel a tap (earthquake!), that will dislodge the governor's arms, allowing them to fall a bit (reducing the oblateness) and temporarily increasing the big wheel's speed. It's technically possible for a tap on the wheel to cause the governor's arms to rise a bit, taking the governor further from equilibrium but that's not very likely; similarly, it's possible for an earthquake to increase the Earth's oblateness but that's not the way to bet.
This refers to the Chilean earthquake (there have been so many recently, I have to specify which one I mean).