It's not unusual for airports to reduce staff at night, and the incident occurred at 23:36 local time. Even at a very large airport in a very busy traffic area, one controller can probably handle normal operations at this hour.

The obvious problem is what happens when operations become abnormal. ATC shouldn't be staffed for normal operations, because then abnormal operations lead to catastrophe. Welcome to last night: the weather is bad, which causes a plane to abort two takeoffs, which causes that plane to need emergency services. This increases the controller's workload beyond his capacity, so he accidentally clears the emergency vehicle to cross in front of a landing airplane, and they can't see the airplane because the weather is bad, so they follow the instruction and promptly get hit with an airplane.

When some bad weather can be the difference between "this is fine, one controller can handle it" and two dead pilots, you need to be staffed for bad weather.