Thank you for the extra photos!
Did some researching and came up with a few possible causes.
They are not normally susceptible to fungal infections, but this could happen under certain conditions.
One, would be too much moisture in the soil which can cause rotting of the roots and eventual death of the tree, or parts of the tree. This is a native tree species that are home to drought conditions so excess soil moisture could be an issue.
Second. the pruning done over the years was not done properly....cutting back branches and limbs should be removed as close as possible to the trunks so the trunk can heal itself over the cut ends! Leaving sections of the dead branches invites insect attack, as well as mold and fungal rot over time which weakens the tree.
Three, mulch is great, but not close to the trunks of the trees. It is not recommended, though gardeners still do it! You need to pull it away from around the trunk about a foot.
I recommend that you to contact a local arborist to have a look at your trees, on how to prune them properly! He may have insight in what is actually going on with this particular tree and can have a look at all the others in your care.
In any case the section of the tree that is dying will need to be removed, but removed in the proper way.