This fungi is a gandoderma.
. There are several types.. some existing on rotting wood and some entering healthy trees by means of a wound and then causing rot.
You probably have the former.
Do you see any "shelf" fungi (gandoderma) on other trees? Or so you remember shelf fungi existing on the maple which was removed. If not, this specimen is probably the species which grows on rotting wood (the base of the maple). It is not surprising that it had taken the trunk and roots this long to decay enough to host gandoderma.
What you see above ground is the spore producing part of the fungi. If you physically remove the above growth as soon as you see it, you might prevent its development but this may be an ongoing proposition as long as there is decaying plant material underground.
I did find the suggestion to apply fast acting nitrogen (not slow release formula) to the area as a way to hasten the process of decay.
From University of California:
Sometimes you can eliminate mushrooms growing from organic matter or hasten decomposition of organic matter by applying nitrogen fertilizer at a rate of 1 pound of actual nitrogen per 1,000 square feet of lawn. The nitrogen should be readily available and not a slow-release or water-insoluble formulation. Examples include 5 pounds of ammonium sulfate (21-0-0) or special lawn fertilizers such as 6 pounds of 16-6-8 or 4 pounds of 27-3-4 per 1,000 square feet of lawn. Fertilization hastens decomposition of organic matter.