How did we get maggots in our pantry?

our pantry is dry and we dont over pack our shelves, everything is either sealed in a tuberware box or storage box and the only things left out side the storage boxes are anything that already come pakaged as boxes (cookie boxes, cake mixes, cereal boxes). we found maggots infesting in the flour, how did tey get there? we never have flies in our house either :(