It was necromancy.
Everyone knows this. The zombies are formed from a necromantically corrupt Mycelium Network.
It didn't take long for druids, wizards, and clerics to get together to figure out the initial zombie infection was tinged with necromantic magic.
In the beginning (Timeline) the Island of Moog was mostly indigenous tribes and small trade posts. It was known to be a home to corrupt wizards, wishing to perform foul magic without oversight or responsibility. The island was believed to be a clean source of untapped mana and pure ether. It was inhabited by primitive locals exploited by such powerful arcanists.
There were eight powerful wizards from around the world. The Arcane Octad. their lairs scattered across the islands, far enough away from each other, it was said, as to not contaminate each others research. They spied and connived against each other constantly, turning tribes against each other and creating armies. It is believed the Octad were killed during an earthquake that ravaged the island, right before the apocalypse began. Attempts to contact them for help (unlikely) were unsuccessful at the time, and they are surely all dead by now. Many think this is a good thing.