Adulteration is one reason. Accidental contamination another.
But also some plants tend to accumulate certain compounds. Possibly to 100...1000x the levels from the soil it grows in. Eg. rice is known to accumulate arsenic occuring in soils naturally. And some soils just happen to be rich in, or polluted with <insert undesired substance here>.
But also some plants tend to accumulate certain compounds. Possibly to 100...1000x the levels from the soil it grows in. Eg. rice is known to accumulate arsenic occuring in soils naturally. And some soils just happen to be rich in, or polluted with <insert undesired substance here>.
Farmers may not even know. Or care...