That seems like rapid growth for such a tree. This may be due to an extensive
root system on the root stock causing the tree to put out a lot of matching
top growth. Plum is in the cherry family, which is normally a hard wood,
but this rapid growth may have caused the tree to develop softer wood.
If you support the tree as I mentioned earlier, the trunk will eventually thicken
up and the tree will be less prone to break. Although it is next to a fence,
if it is upwind of the prevailing winds in your area, it will still take the full
impact of these winds. If you lost the limb because of too much fruit on
it, try thinning more aggressively. Another possibility is some animal, like
a deer breaking the branches or pushing the tree over.

Sherwin

wrote:

I like the tree, and I would like to save it. I planted it last spring
as a four-foot stick, and it's already grown to about 10 feet tall and
borne fruit once. Is that abnormally fast growth? Would that help
account for its breaking? It broke below the graft, and it's next to a
fence, so it's not really unprotected. It lost a limb earlier this
year, so maybe it's weak-wooded.