What's all the ground cover stuff growing under it?
Could you get a few more pics of the trunk/ground area?
Lets see how close that tree is to the retaining wall, also inspect the wall from road side, maybe a shot of that to.
See, the tree only has maybe 60% of it's root entitlement due to location (wont grow roots in fresh air). If the wall has cracked allowing moisture table to drop etc it could have impact upon a mature tree.
Now the probability of canopy issues usually
but not always will coincide to the side of the tree with root problems. Is the road side of the tree canopy worse than the opposite?
So with a reduced root entitlement, any competing ground covers etc will also have more impact, has the surrounding shrubs, lawn, ground covers had a blooming good year too?
Finally, in restricted root areas, where trees get hemmed in by structures, similar to a plant that gets pot bound they can start to struggle as they consume resources and have less new ground to grow. Maybe time to re-assess around the tree to replenish it's growing environment. You know, clear around it, mulch, soil condition with organic stuff like blood/bone,
(do not fertilize with aggressive stuff loaded with nitrogen).
Also, if you can get out your spade or hand trowel and dig a little hole, maybe 10" deep, fill it with water and see how long it takes to drain. When you dig the hole observe the roots, do this say 10' plus away from the trunk, you should find quite a few fine hairy roots in the soil, especially the top few inches. And just while you are there perhaps a PH test, make sure the soil is OK. You could do this in perhaps 2 or 3 spots.
Well, have fun, post your findings and pics.
