I had trouble finding articles detailing how the nuclear plant was brought down, but then I found this:

> On Monday, Feb. 15, 2021, at 0537, an automatic reactor trip occurred at South Texas Project in Unit 1. The trip resulted from a loss of feedwater attributed to a cold weather-related failure of a pressure sensing lines to the feedwater pumps, causing a false signal, which in turn, caused the feedwater pump to trip. This event occurred in the secondary side of the plant (non-nuclear part of the unit). The reactor trip was a result of the feedwater pump trips. The primary side of the plant (nuclear side) is safe and secured. […] We evaluated Unit 2 and have confirmed that we do not have the same issues that caused the feedwater pump trips in Unit 1.

and subsequently, an answer to my question of 'how':

> Some people have wondered how “pressure sensing lines” for a feed water pump could have been affected by cold outside air temperatures. There are no turbine halls at STP, both of steam turbines are out in the open air. I’m sure there is a design reason for this choice, but it isn’t apparent.

(from https://atomicinsights.com/south-texas-project-unit-1-trippe...)

Water related to the main cycle of the plant? Or something non-essential like the toilets, or some valve froze shut because water froze on the outside of it?