Hello Casey

Scars from a couple years ago won’t tell us anything about the current health of the tree. If you are noticing thinning this year only, with no other symptoms( such as new exit holes, sawdust at the 

base of the trunk, oozing sap, completely dead branches)  there are a couple of reasons I can think of that could be at issue. First, Michigan had a hard freeze the first week in May and this killed some early buds and leaves on Maples. Second, it could be a problem with past droughts. The effects of drought will show on the trees several years later. 

A complete assessment and health care plan by a certified arborist may be in order if this tree is high value. The cost is relatively modest compared to losing a large mature tree and having to remove it. Certified arborists are professionals who have been educated, and then passed certification tests, in pests, diseases and tree health care. You can find certified arborist by ZIP Code here- www.treesaregood.org

What you can do is have a professional prune dead or diseased branches correctly; slow, deep watering of the tree during hot dry droughts in summer and in a dry fall; and continue to watch for insect activity, such as new exit holes  or honeydew dripping from the canopy. Protect the root zone by avoiding foot traffic  and not mowing over exposed roots- instead mulch with wood chips or shredded bark 2-4 inches deep in a circle and not piled against the trunk.

Here are references for you

https://www.canr.msu.edu/news/the_dos_and_donts_of_irrigating_landscape_plants

https://extension.psu.edu/maple-diseases

http://www.uky.edu/Ag/Entomology/treepestguide/maple.html

Hi Casey,

This is some sort of canker or slime flux on the tree, caused by fungi or bacteria. Your best action is to hire a certified arborist to assess the tree and give you a care plan.

This cost is modest compared to losing the tree and having to remove it. And of course, you can not replace a mature tree in your lifetime! So, give this tree its best chance by getting an exact diagnosis from a certified expert. ( not a tree service that just cuts down trees) See the first answer above, to find them.
Here are references for you on some of these conditions- https://extension.psu.edu/bacterial-wetwood-or-slime-flux

https://ag.umass.edu/landscape/fact-sheets/target-canker-of-hardwoods

https://extension.psu.edu/eutypella-canker-on-maple

https://ag.umass.edu/landscape/fact-sheets/phytophthora-bleeding-canker