If there are no signs of fungal disease on the tree, and I can't see any in the photo, chances are that your maple has experienced sun scald.  This can happen in late winter or early spring when there are warm, sunny days followed by freezing nights.  The warm sun shining on the trunk causes the tree tissue to break dormancy too soon. Then a freezing night causes the tissue to die.  The tree can produce scar tissue to heal this wound just as it would when a branch is cut off.  If the wound is too large, then the tree probably won't be able to seal off the damage.  Then fungal and or insect diseases can probably take hold.   I can see scar tissue in your photo and you haven't seen signs of disease, so presumably your tree has recovered.  The tree will not be able to replace the missing bark however.

As a side note, it is never a good idea to put mulch right up to the trunk of a tree.  This is because the mulch holds in moisture which encourages fungus to grow.  Keeping the mulch a few inches away from the tree will prevent fungus from infecting the bark.   

Read more at Gardening Know How: What Is Frost Crack: What To Do For Cracking Tree Trunks https://www.gardeningknowhow.com/ornamental/trees/tgen/cracking-tree-trunks.htm

Gretchen 

Thank you for your detailed response.  

Regards,

Dave Haldeman