Seth's voice paired with his word choice immediately put Francis on edge. He had been fully expecting to attempt to console Seth a little and say some genuine nice words about Clark Enders, who Francis had looked up to since he could remember. However, now that Seth was implying, and strongly implying, that the scientists had made up the heart attack story, Francis could think of nothing else.
If that was the case, it meant it was power-related - something to do with the project that other people couldn't figure out. Francis hadn't been told anything, no one had contacted him. If Seth was really saying that something was being covered up, someone had forgotten to tell him.