It looks like the bark on the trunk of the affected limbs is peeling off. This would seem to indicate that the problem started several years ago...and that would be typical. Most often we don't notice an issue with a tree until there are obvious signs (leaf loss, lots of insect activity, wood peckers, etc). But these signs are usually secondary to the initial issue.

Yellowwood trees are susceptible to verticillium wilt (a fungal disease) and borer damage. Ambrosia beetles, for example, are a type of boring beetle goes under the bark, leaving wet areas on the bark or frass tubes. I can't tell from your photos if these issues are present. Soil compaction can also cause stress to trees and signs may take years to manifest...so also consider what may have changed in the last 5 or even 10 years in the area around the tree. 

The white/light green patches on the trunk are harmless organisms called lichens.

If these trees are very important to you, you could hire a certified arborist to come on site and evaluate them. An arborist would have the equipment and knowledge to do a site assessment, check for borer damage, and take samples from the tree to a lab to determine if disease organisms are present. You can find a certified arborist near you using this page from the International Society of Arboriculture, https://www.treesaregood.org/

Hope that helps!