To me, it's just stale. It's made way in advance, then stuck in a bunch of bags, or a big bin, and just sits there, for god knows how long.
Even at one of the biggest theatres in London, I thought, yes, I'll get NICE popcorn here... when I asked, they pointed towards what looked like an old fridge at the side (it wasn't cold, but had shelves and a clear door, hence my comparison), full of popcorn bags. I was just supposed to grab one for myself. It tasted like it'd been there for hours.