CompTIA Server+ SK0-005 Practice Question

A systems administrator is troubleshooting a 2U rack server that has begun shutting itself down without warning during nightly backup jobs. The baseboard management controller reports the CPU package temperature climbing past 100 °C within five minutes of power-on, even when the operating system is at less than 5 % utilization. All system and CPU fans immediately ramp to maximum RPM, yet the administrator can feel almost no exhaust airflow at the rear of the chassis. After removing the lid, the administrator sees a thick layer of gray fluff covering the intake grills and heat-sink fins. No firmware, driver, or workload changes were made recently. Which of the following is the MOST likely root cause of the thermal shutdowns?