Trados Studio 2021: Status of every segment changed from Not Translated to Translated

About a quarter ways down in a document with 870 segments, I noticed that my progress (in the lower right) showed 0% Not Translated (it was showing the correct percentage moments earlier) which made me realize the status of every single segment had magically changed from Not Translated to Translated, but with no content in the target segment.

Is there some keystroke combination that can create a situation like this or do I have a ghost working over my shoulder?

I easily fixed this with the Advanced Filter (Filtered Translated segments changed the status to Not Translated) but it was just so strange.

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