What a shame. Did you get a sudden douse of rain lately? - I know, that seems like a silly question given that you are in California.
Our citrus trees here have their root systems very close to the soil surface, so any sudden rain or increase in watering is absorbed quickly, which according to your excellent article might have been what happened.
To prevent this happening again, I would apply some mulch to the root area, to help keep the soil moisture more consistent. Spread it thick, and out at least a foot beyond the reach of the branches (drip line) because the feeder roots go out that far. Then be sure to water as regularly as you can for the same purpose. The only caveat on the mulch is to keep it away from the trunk 6in. or so. Here we get trunk or foot rot easily if the trunk of the tree stays too wet. Again that doesn't seem too likely in CA, though.
