Why are some leaves (near the end of some branches) dying on this Red Blush grapefruit? Most of the tree looks perfectly healthy. One branch that had this leaf die-off dropped all its grapefruit in one day. What's going on?

Perhaps a stink bug or similar juice sucking bug got a field day on that succulent branch. Happens often on my roses and I caught the critters. It could also be similar to glassy sharpshooter type of juice suckers.

Have you checked to be sure the branch hasn't been broken, or have you checked for spider mites? - Millet

I don't think it's related to insects. We don't have trouble with spider mites at this time of year. They come out in April and don't cause this type of damage. The branches were not broken and the leaves were perfectly healthy only a few days ago. This is weird.

Last year, another tree (of the same cultivar) on the other side of my house did the same thing. About a month later, the branches where all the leaves died off were covered with blooms. The leave die-off was a prelude to exceptionally heavy blooming.

A week or so ago the branch was healthy. Here's my tree from another angle - the branch in question is on the upper right. As you see, the leaves look okay.