They run fine in a lot hotter weather in the summer so it wasn't the heat alone that did it.

The ERCOT grid in Texas is pretty much isolated -- there is limited transmission capacity to import power from (or export power to) neighboring grids.

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Source: https://en.wikipedia.org/wiki/Texas_Interconnection

Thus a failure may cascade into many failures and that will really affect reliability. They overbuilt on wind, for example, but if the wind isn't blowing when they need it the rest of the generation capacity may not be able to make up for the difference. I have no idea if lack of wind was a contributor this time but it has been a common denominator in previous ERCOT grid reliability incidents.