agreed - maybe plum...maybe crabapple.
are there as many suckers coming from the bottom of that thing as it looks? and where on the trunk are you finding green? I'm wondering if that main trunk got girdled by a borer of some sort, down where it was protected by all the other growth. the higher you're seeing green on the trunk, though, the less likely that's your answer... also, in general, leaving suckers to grow around the base (especially on a grafted tree) can weaken a tree, since the roots are sending up their own top-growth...and the more they send up, the more they feel they can let the main trunk die, since the roots are finally getting a chance to have their genetics expressed.
kind of weird way of explaining that - hope it helps.