In addition to others comments...
Are we looking at Acer? I do not see any Taper where the trunk meets the soil line, and as stated before looks like volcano mulching. Is there trunk taper at the "Soil Line" when you pull back the mulch or are the trees still planted too deep?
Do you know when the trees were planted? For each 1" D.B.H. caliper at planting time, the tree will need 1 year to establish rooting to resist drought stress. Example a 3" cal. plant at planting time will need correct watering for 3 seasons....Client education a must here.
Dieback of the leader can follow a spring that favored Anthracnose with excessive leaf drop, or if the soil is too wet for too long a time......Poor Co2/O2 Exchange in roots, just to name a few causes.
I would suggest you consult with a "Certified Arborist" from your area and spend the couple hundred bucks, and learn from a pro in person.
Money well spent, You will be a more valuable professional, and your client will have peace of mind.