A nearby GDI base is conducting further ion research. In doing so, they use a large number of chemicals, located in their bio centers. Destroy all the bio centers, and "contaminate" all units and civilians. Leave all the other structures intact, so that it looks like an accident. - Mission briefing
One of the strengths of Nod during the First Tiberium War was its control of the media. The Białystok scandal demonstrated how Nod would spin the media against GDI by causing destruction in their name. GDI conducted a broad range of dangerous tests during development of their Ion Cannon. This gave Nod another opportunity to damage GDI's public image. Sending a small force into GDI territory they planned to destroy the volatile bio centers. This would cause massive environmental damage to the surrounding area, creating a PR nightmare for GDI.