Without any warning or apparent reason, a catatonic patient suddenly strikes a fellow patient, and afterward is unable to give any explanation for why he did it. What is this?

This is impulsive behaviour: a sudden action, typically aggressive, carried out without warning, planning, or forethought, with no discernible motive identifiable after the fact, and common enough in catatonia, exactly as described. It is not extended suicide, since impulsive behaviour differs from extended suicide, which, despite superficially appearing sudden, is typically driven by a specific, sustained delusional belief system rather than arising without any identifiable motive. It is not negativism, which is active resistance to a requested action, not a spontaneous, unprompted attack. And it is not impulsive personality disorder, which is a lifelong trait pattern with a characteristic build-up and remorse, not a single isolated act occurring within catatonia.