Unless the bark on the trunk has been girdled (removed) most of the way around, or damaged in the same way by a weed whip, the most probable cause is stem girdling roots.  This is very common in maple trees planted too deep. 

Girdling Roots (missouribotanicalgarden.org)

You may have to have a certified arborist make an examination.  It will require physically cutting and removing those roots.  You can find an arborist in your area by going to this link and putting in your zip code: 

TreesAreGood.org