I seem to remember the last time this happened the culprit was discovered to be people stealing CFC-containing waste tars from the factories they worked at, the

I seem to remember the last time this happened the culprit was discovered to be people stealing CFC-containing waste tars from the factories they worked at, then selling them on for use in bitumen roads. Iirc, CFCs are a unintended byproduct of a lot of industrial organic chemistry that uses chlorine - so there's an unfortunate mismatch of incentives between factories, which wish to reduce costs, politicians that want to reduce costs, and thus regulation and/or regulatory enforcement, and people who don't want a nice new melanoma.