12am-5am is very quiet, at about 1 per hour. But the accident happened during the 10pm-12am time slot, which is not as busy as other times of day, but can still have workload spikes as evidenced by this situation.

ATC should never work alone at any of the "Core 30" airports. https://www.aspm.faa.gov/aspmhelp/index/Core_30.html