Hard to judge a species or cultivar by what a single seedling/sapling does, but some selections may be weak. Straight S maples are tough, persistent & difficult to kill (seedlings keep sprouting back after cutting down to the ground on my lot).

I pulled away the mulch for a couple hours on a particularly hot day to fix the grade

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what exactly did you do.. to fix the grade .... slice away the topsoil to lower it ... or add to lift the grade??? .. how much root damage could you have done ... ???

i often .. perhaps endlessly .. lol .. talk about tree time ... im just surprised the leaves turned brown .... in a couple hours ...??? .. reaction to stress.. usually take a lot longer than that.. in the tree world ...

ken

What does "fixing the grade" mean? Altering the soil level surrounding a tree can have some pretty serious repercussions. As can exposing or disturbing a lot of the fine feeder roots unnecessarily. That's how pathogens like Verticillium wilt get transmitted. And all maple species are overly prone to VW, which can kill a tree almost overnight.

Sugar maples are a very tough and adaptable native species and not the least bit finicky, which is why they are a dominant feature in much of the northern and midwestern hardwood forests.

It may be just that your attentions, however well intended, may have exposed the tree to some sort of issue that hastened its demise.

I added a couple inches of soil, flattening the slope some. The original planting job was a bit wonky. True there likely was more than one issue and exposing feeders might have pushed it over. And yes half the tree went brown in the afternoon I worked on it.

Poorly Planted Sugar Maple Did Not Make It.

There, I fixed it for you.

Mens, no just a bit too much slope, could have been left alone, yet I obsess; and the tree this season looked the best it has been with finally some vigor in the leaf out.