Roughly one per year. That is probably below the historical average.
Ammonia is dangerous stuff. There are industrial accidents involving dangerous gasses on a regular basis.
Sometimes accidents are just accidents. Terrorists want us to know they have struck.
Followup from local radio (WSB AM) is that a forklift operator hit and ruptured an ammonia pipe. It's just the usual warehouse carelessness, unfortunately. Sometimes they hit a sprinkler pipe . . . but sometimes it's something nasty like ammonia.
VC, my husband works about three miles cross-wind from this establishment. (I called him to make sure things weren't blowing his way.) He works for a chemical company, says he's glad they don't mess with ammonia much.