I'm unsure the make/model of your box, but it reads like a Power Saver preference. I agree it's baffling why the preference would suddenly start or enable.
What's suspicious is the audio continues. If a Power Saver is to engage, why would the audio continue? Perhaps it's your TV set is going into a Saver mode. Again, I understand you wouldn't have recently enabled it. Perhaps the TV auto-updated it's software and defaulted this feature.
Mostly the box is one one national TV news channel, sometimes on another. Never on local TV channels.
Good addition to your question because this should rule out any Switched Digital Video channel.
