One possibility: a screw-up by the local affiliate. I've run into similar issues a number of times (although not lately) - for a whole, the local CBS affiliate had a bad habit of taking a Dolby 5.1 source and only broadcasting the left and right channels for the HD feeds. The result was a Dolby 2.0 track with no dialog. Errors like this rarely affect the analog broadcast, as they've had the bugs worked out of that system for decades.