Over-spray by some herbicide - like RoundUp, is my guess. Too many people spray when they shouldn't, like a windy day.... and it does carry.

Where do you live and are you in an HLB quarantine zone?

If they are Valencias, they wouldn't be ripe at this time even though the peel is orange.

Last year we had a very wet winter, but it hasn't rained much since spring. Did you keep up with regular watering?

We're just north of SF in Marin Co., so not in a quarantine zone. This is a Naval Orange. Last year this time we had lots and lots of orange juice to freeze.

Probably a little hit or miss on the watering, but not totally neglected. Actually, this tree has lived a somewhat neglected life. Until we moved in 2.5 years ago, the homeowner was an elderly lady who didn't go out and water it. At one point there was an irrigation system that covered it, but that system had pooped out by the time we got here. Maybe the gardeners watered it some? Regardless, the tree looked and performed well for the first 2 years we've been here and it's demise has been sudden.

It looked good in Oct before we took a trip out of the country, lots of fruit setting. While we were gone there was that huge fire up in wine country, but we're far enough away (20 mi?) that is unlikely as a factor. In Nov. it looked a little sad, and by Jan. it looked desperate. It's now officially dead.

Autopsy: we cut the poor thing down Tuesday. DH took branches with leaves and bits of roots to the UC Extension people (Marin Master Gardeners). They looked under a microscope, but didn't see anything there besides definitely dead. They shook their heads, discussed it some more, and didn't come up with anything. A mystery.

Pics look like it got too dry. The clues: last years wet season vs this one, irrigation system kaput. Many people fail to give mature trees an adequate deep soaking that they need. Citrus is a tropical tree, so they need water - lots of it!

Then again plants and trees sometimes just die suddenly!

the same thing happened to my meyers lemon tree last year, beautiful healthy tree then the leaves started to curl, turn yellow then all the leaves fell off . I think the neighbors gardener may have let his Roundup drift to my tree because along the fence line the grass is dead and another one of my plants in a pot is doing the same thing this year, but I can only assume this is what happened

RoundUp-type products kill s-l-o-w-l-y but surely.

I doubt its roundup or similar product. It would take a good heavy and high spray to get a mature tree to die. And likely more than one application for a tree of this size. Is it possible, sure. But i doubt it.