An owner of a local TV station (WDBJ) posted this on a locally oriented AVS forum.
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A new FCC regulation will require local digital stations to change the way "PIDS" data is inserted into the signal effective January 1. With short notice, most stations are scrambling to make the deadline -- and most won't get it done this week. Changing the way this data is inserted will mean your receiver will no longer be able to understand the signal it is getting until it "re-learns" the station. Each local station will have to make modifications to its encoder, and each is likely to do so on a different day. That means you may well see some stations you have been receiving just disappear over the next couple of weeks -- one at a time -- until your receiver scans again. There is also some concern that certain older ATSC receivers may not be able to decode the new data stream at all.
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This might be what is happening.