You may want to reconsider how you are hiding the container components too. No need to keep hiding them over and over when you could just do it once. Also, there is more than one BackgroundMiddle texture to hide.
Code:
------------
--Hide Bag Backgrounds
------------
local function Hide(frame)
frame:Hide()
frame.Show = frame.Hide
end
for i = 1, NUM_CONTAINER_FRAMES do
Hide(_G["ContainerFrame"..i.."CloseButton"])
Hide(_G["ContainerFrame"..i.."BackgroundTop"])
Hide(_G["ContainerFrame"..i.."BackgroundBottom"])
for bg = 1, MAX_BG_TEXTURES do
Hide(_G["ContainerFrame"..i.."BackgroundMiddle"..bg])
end
end