What would cause healthy appearing limbs to break off my black walnut tree?

Question

Our very mature black walnut tree has recently lost a few branches and limbs that appear healthy with abundant green leaves and fruit. The tree was trimmed a year ago. There have not been any destructive storms.

Answer 1

There are several possibilities.
If the pruning was extreme crown reduction, last year or sometime in the past, with stub cuts that re-sprouted, it is common that the re-sprouted growth is not well attached and will develop structural defects and potential breaking points. There are other reasons for defects:
major crown reduction - radical topping that exposes the woody stems to sunlight will scald the bark and kill underlying living tissue which progress into wood decay and potential breakage.
Over burdened branch end-weight is sometimes neglected by tree workers; the end-weight can create susceptibility for breakage.
Nutrient deficiencies - example: Calcium is important for cell wall strength and therefore structural integrity. Boron is important for Calcium metabolism. These minerals leach from the topsoil with rain and irrigation. Soil fertility is a vital consideration.
Summer branch break phenomenon. Foliage and wood is at its maximum weight from water uptake in spring and summer. This added weight can predispose the branch to breakage even without defects.
Here is more:
https://www.gardeningknowhow.com/ornamental/trees/tgen/dry-brittle-tree-branches.htm