Eg: N2(colourless gas)+O2(colourless gas)=NO2(brown coloured gas)

In my parent's house, we used to store some plastic salad boxes in a wooden cupboard in the cellar. Before storage theses boxes have been emptied, washed and stacked into one another. After some time, at least half a year of longer, we found some of these boxes glued together, where their walls touched each other. To me it looks like there was a chemical reaction going on. Can somebody please explain to me what exactly is going on there?