Hi Dorothy:
Did you recently complete construction on a deck project in your backyard, adjacent to where these trees are located? If so, were these trees and their associated root system adequately buffered from any heavy equipment (soil compaction) and excavation (damage to root systems)?
Tree decline and death can occur 3-7 years after local construction projects, so this sort of mortality can be common if proper tree buffering controls were not properly envisioned and enforced.
My only suggestion is to have a professionally trained arborist examine the trees on-site for signs of decline and mortality.
IL Certified Arborists:
http://www.illinoisarborist.org/CertifiedArboristsNew.htm
Best of luck!