Arborist here (retired)
Lots of things to consider, and almost impossible to diagnose without a site visit.
That having been said, there are a couple of ideas I can throw out there for your consideration.
You could have a massive infestation of "boring, chewing insects" typically beetles, that just showed up within the last year and completely severed the vascular system just under the bark that feeds the tree (Cambium layer). Look closely at the trunk of the tree for bore holes, fine sawdust. Look up the trunk as well as ground level. But it's unlikely that the same type of beetle would be infesting those two very different types of trees.
Is "oak root fungus/armilleria " prevalent in your area? If so, you may have noticed large masses of mushrooms clustered around the trunks and root systems of the trees during your rainy season.
Could just be that both species have just "aged out".
Typically though, it's a number of things each of which have an impact on the tree and will add up to decline and eventual failure over the course of a number of years.
Construction damage comes to mind right off the bat due to your description of the trees location.
They don't like grade changes too much. Was soil heaped up around the trunks? Heavy equipment parked on the root systems (soil compaction)?
Roots cut for driveway installation?
Concrete driveway? That will burn the roots pronto.
Cement truck driver do his wash-out after the pour in the shade (right on the roots)?
Lots of things to consider.
I would also take a look at the other trees in the neighborhood as you're driving around.
How do they look?
