Greetings Tom:
Unfortunately, many tree/forest health questions are just too general to answer without an on-site assessment. It may be biological (pest/pathogen), it may be environmental (weather related phenomena, pollution, soil compaction, etc.), or it may be a combination of biological and environmental agents.
If you really want to know what is going on with your tree, I would strongly urge you to contact a local professional consulting forester / certified arborist for an on-site assessment.
IL Professional Consulting Foresters:
http://dnr.state.il.us/conservation/forestry/
IL Certified Arborists:
http://www.illinoisarborist.org/CertifiedArboristsNew.htm
Best of luck!