Probably the corn seeds were inadvertently mixed in with the soybeans, but it's possible someone was trying to import them illegally. Translation of news article:
"[On January 11, 2010], the Binzhou [Shandong] branch of the Inspection and Quarantine Bureau found corn seed in a shipment of U.S. soybeans. Testing confirmed that the seed was genetically modified. Material from this shipment of soybeans has been destroyed according to regulations."
"It was learned that this was the first time genetically modified corn seeds were intercepted in soybean shipments by the Binzhou bureau. If this genetically modified seed entered China it could carry diseases like bacterial wilt, maize downy mildew, maize chlorotic mottle virus that could have serious effects on our country’s corn production. Binzhou Inspection and Quarantine Bureau activated contingency plans, carrying out strict quarantine and supervision of four companies in the region that import soybeans to prevent outbreaks of foreign diseases and pests."