Final answer:
The most plausible cause for the observations is a DDoS attack.
Step-by-step explanation:
The most plausible cause for the observations described by the server manager is a Distributed Denial of Service (DDoS) attack. A DDoS attack occurs when multiple compromised computers overwhelm a server with traffic, causing it to become unresponsive. This can lead to increased resource consumption, unusual system behavior, and increased network traffic that is not proportional to the workload on the server.