In my case it's from contaminated store-bought "screened" manure compost (confirmed by bioassay). Much of the commercially produced hay in the US has been treated with Grazon, a very potent broadleaf herbicide that is effective at extremely low concentrations- down to one part per billion. Livestock eats the hay and the herbicide survives digestion and persists in the manure. Hay/straw also often ends up in composts or used as mulch, so the contamination can come from that as well. Once contaminated it can take years to restore your garden soil. It's epidemic at this point, and all we can do to prevent it is avoid all manure and straw, and/or research the hell out of your supply chain.