Japanese maples are marginally hardy in Minnesota. The last few years have been extremely dry. If you were not offering supplemental water (about 1"/week) this could cause decline. How deeply the tree was planted can cause decline if done too deeply. This causes the roots to circle below grouond The decline could also be from disease. Below is a blog link that gives supporting information on what may be going on. 

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=31315