@cornered most movie houses don't pop their own popcorn - it comes from a distro center.
Worked for a theatre chain for 3 years, they popped the corn at the main warehouse where all the old standees and props were stored. We'd order giant bags of corn for the whole week, and they'd get delivered on Thursdays. To refresh, we'd push hot air thru it, in the hopper you see them scoop it out of.
Bet someone forgot to close the bag, or it wasn't warmed up yet. :\