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.
Curious why the back up power didn’t come on till 90 minutes later though… seems like to me a long time if you’re supposed to be in constant communication with a crew aboard a space craft.
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.