Hi Chef..This can happen occassionally w/citrus, but there's usually a reason.
For instance, have you done anything like repotted or added a chemicals? Repotted?
How about soil. By chance, did the soil dry out too much or kept too wet? Either under or overwatering can cause leaves to drop.
This is a long shot, but it pays to be cautious..have you checked for plant bugs? Most don't work that fast where every leaf falls off a tree, unless it happened over a period of time. Toni

Citrusboy, what size pot is your Yuzu in? It looks huge, but so does the tree. How tall is the tree? Toni

Hi Toni,

18" Pot from Home depot. That Yuzu is about 40" tall and has been getting wider and not too much taller. It's on dwarfing rootstock. The two in the ground seem to be growing at just about the same rate.

Last year I grafted some Sudachi budwood to the Yuzu in the Pot and 1 in the ground. Both budded and are growing nicely. Downside is they barely set fruit this year. 2 fruits 2 trees. The 1 tree I didn't graft has 10 Yuzu fruits.

Hi Citrus,
I wonder why your Yuzu is growing wide instead of tall? I have one citrus doing the same thing..debating whether or not to clip it off..what do u think? It's a variegated Pink Lemon.
I first thought it was rootstock since its location is lower on the tree, but above graft line. What are u doing about yours? How much longer is the long branch compared to other branches? Did you prune? or will you wait till spring? Toni

Hi Toni, I don't prune anything except my lemons and my 3 standard trees (october). Seems they all round themselves out over time. Limequats get pretty leggy too but I never prune them. Want as much fruit as possible.