Hi Ho's response above is right on the target. As long as the regular program is in the correct aspect ratio, then the phenomenon you describe is coming from either a cable company, or a local station, or a network. In some cases, it is actually a commercial which was shot in widescreen which was then re-formatted by a station to be sent out on a non-widescreen channel... (for example, an older analog local channel). Then THAT version is re-routed back to a digital channel which is running widescreen content... This can result in bars all around...
In other cases, it can simply be a case of some engineer (or some babysitter) in a control booth who forgot to hit a button, and widescreen material gets re-formatted for one or two commercials, during a break.
I did want to add to the discussion in one way -- there is a term which I have seen to describe this type of image. On a few of the forums, at least, I have run across the term "windowboxing" to describe this sort of picture.
-Bruce