It happens when the tree is stressed out most of the time due to water needs. How the leaves look?

Some of the leaves look dry. I wasn't sure what was happening, because it's on a drip system. I've given it extra water a couple of times. I know it's getting some water, because there's water in the plate under the tree, but I don't know how much water it's getting. Maybe there's a clog in the line.

If the pot is soggy, the roots won't get air. The result is the same as too dry.

I'll check the pot again, but I even had the yard guy put a few more holes in the bottom of the pot this year. I didn't want it to get waterlogged, and it's in a plastic pot. Our area has very hard water and the lines often get clogged with calcium crystals.

All my pots have several holes on the bottom and at-least 6 inches up the sides of the 1/2 wine barrel containers for drainage. In my experience the more drainage holes the merrier the trees. :)