So I started re-reading the Dragon Heart trilogy. The explosion itself still seems to remain a mystery, but it does seem to be directly caused by Big D himself. The Dragon Heart is a very powerful relic and from what I can tell from reading and remembering (haven't finished my re-read) it couldn't be activated with the level of magic at the time, and maybe not for several more decades, and needed a massive mystic boost to get it running.
The lack of radiation or other explosives chemical residue does point at the destruction being magic in nature, the perfect circle of safety shows that he had prepped from explosion so it wouldn't hurt the populace at large. It may also have worked to focus the energies inward and direct them, making sure they both killed him and were directed into the Dragon Heart.
I'm thinking Dunkelzahn may have used a type of necromantic metamagic to harness the energy of his own self-sacrifice to give the ritual a boost. Kinda dark and bordering on blood magic, but he was apparently desperate, especially since The Laughing Man just left Thayla on her own with no real backup against the horrors and their human followers on the near complete spike point. His death got the Dragon Heart up and working, and ironicly thanks to Thayla apparently calling up his fragmented spirit (guessing since he was clueless as to who he used to be), he ended up personally using the Heart's power to smooth out the spike points like a giant astral belt sander.
Dunk planned the whole thing including his "death" and the events that spawned from his will. The only reason he stooped to becoming the president of the UCAS was to tap into the magical power that the title has.
Not this crap again. Joe if you're going to talk from an in game perspective take it to the RP forum. Show me an example, in canon, where name magic exists in Shadowrun or where one of the game-makers has said so. Just because a form of magic existed in Earthdawn doesn't mean it exists yet, or ever will exist at all, in Shadowrun.