You can only wonder what properties the spores in the mushrooms have that can make someone this spooked...
Earlier, the original owner of the Fungeon was struggling financially,
and failed to pay the rent to the Dungeon Broker,
who trapped him in the Infinite Dungeon with very little to fight with,
and, later, proceeded to sell the Fungeon to someone else,
while the former owner is still trapped in the Infinite Dungeon.