Genetically modified (GM) crops are foods that have had their genomes modified, allowing humans to increase the production of certain crops. Farmer A purchases GM seeds that are immune to a specific herbicide. She plants these seeds in rows over seventeen acres. The following spring, Farmer A's neighbor, Farmer B, tells her that nearly a third of the plants in his crop fields have the same immunity.

Which of the following describes the most likely explanation for this occurrence?