NASA loses contact with ISS after power outage in Texas, uses backup system for first time

They have most certainly done drills and tested the backup. This was probably the first time it was used for real.

The incident was linked to upgrade work that was underway in the Johnson Space Center’s building.

Backup systems usually fail over during regular outages. But during work, they might have had some stuff disconnected for access or safety. Or some worked accidentally tripped something that caused a failure in a weird way. Or it was a brownout that wasn't enough to trip the failover.

The backup is in Colorado. You keep your primary and secondary at two different geolocations in case of an emergency. 90 minutes to bring a system online and communicating is pretty good.