Why Humanoid.Died fires multiple times?

Hi. Humanoid.Died event fires 3 times but why this is happening ? I fixed it with debounce but i would like to know reason.

local npc = script.Parent
local hum = npc:WaitForChild("Humanoid")

local debounce = false

hum.Died:Connect(function()
    if debounce then return end
    debounce = true
    print("Fired!")
end)