There are so many factors that could be impacting an older oak tree. It is likely a combination of factors.

Have you treated your yard with any fertilizer or pesticides recently? Or are there any other factors that have changed in the zoot zones of the tree (root cutting, soil additions, soil removal, construction compaction)? These are questions an arborist will ask.

An older tree of this size located so close to a property should be looked at by a trained arborist. Minimally you should search for an ISA (International Society of Arboriculture) certified arborist. You should also consider looking for a TRAQ certified arborist to do a tree risk assessment (TRAQ is an ISA qualification program that trains arborists how to use methodologies outlined in the ISA Best Management Practices for Tree Risk Assessment). 

Click on the CLICK HERE words in the link below and follow the prompts to find one in your area.

Resources - ISA Michigan (asm-isa.org)