But why is Kaja'mite most commonly found underneath the volcano on Kezan?
The volcano has presumably been there at least since the primordial age when the elementals waged war on each other and caused the land to constantly change. I don't think the Sundering would have caused a volcano.
Did the Kaja'mite flow out from the planet's core with the magma?
Were the magma tunnels once home to a part of Y'shaarj?