Many cable channels are created to fulfill a specific programming niche, and their name is Exactly What It Says on the Tin — the Golf Channel shows golf, the Game Show Network shows Game Shows, and so on.
Some channels, however, are not as wedded to their original concept as others. In a pattern known as channel drift, meddling executives look at the Demographics to whom their channel appeals and decide that, hey, since the people watching their Speculative Fiction channel are mostly 18-31 males, and Professional Wrestling is hot among that demographic, surely no one would mind if they started showing pro-wrestling! 
The fans of the original programming will mind, of course, but the channel tends to keep going regardless. This may show up with only a couple of odd programs in the schedule, but far too often, given enough time, a channel will have pretty much abandoned its original concept. Whether or not the former invariably leads to the latter is a subject for debate.
Since the network is strongly impacted by the ratings, and the highest ratings go to generally the same few demographics, this tends to lead to networks becoming more and more like each other, either in similar programming or outright airing the same shows.
Some changes can be chalked up to the changing landscape of TV. As the number of channels goes up, networks re-align themselves to try and hold some of their market. That, or the parent companies who might own seven or more cable channels each shuffle stuff for "synergy" or to reduce redundancy. Competition with new media is prevalent as well—classic reruns give way to DVD box sets, music-video channels give way to YouTube and iPods, and info-dumping all-text channels give way to the data display in a digital cable box or some new-fangled webernet site. Most of the time, it's just shifting to whatever the network feels will attract the biggest audience--and the audience that lets them charge the most for ads (especially the lucrative young adult demographic, needless to say).
While in many cases this will often cause a Broken Base among the channel's viewers, and can even kill the channel’s relevancy completely, there are instances where changing concepts could actually be a good thing for the channel. If the Network Decay works out, it may expose the channel to thousands of new viewers, who would normally never watch the network in the first place. Or perhaps the earlier direction just was not working out and the network made changes in order to get better and more profitable programming. Furthermore, there are several good shows floating around in Development Hell that wouldn't stand a chance of getting picked up unless a network decides to spread its wings.
See Magazine Decay for the print equivalent. See also Artifact Title, They Changed It, Now It Sucks, Screwed by the Network, and Adored by the Network. Public Medium Ignorance may be a cause for some of the listed networks. If it starts overlapping with politics, then it can cross over into Strawman News Media.
- Total Abandonment—The channel, with the exception of perhaps a few shows, has long abandoned its original concept.
- Slipped—The channel still shows programming related to its original concept, although it is significantly showing programming not related to their genre in some way.
- Major Shifts That Fit—The channel has been pushing or narrowing the limits on what they can show without leaving their genre entirely and/or are being too over reliant on a Cash Cow Franchise or two, but at least what they are doing still fits the original mission somewhat.
- Temporary Shifts—The channel, after decaying for a while, has since returned to its roots and are showing programming related to their original mission again.
- Unique Situations—The channel’s decay either is in a situation where it doesn’t fit any of the other categories, or is constantly fluctuating between decaying and recovering.
- Notable Aversions—The channel has managed to avoid decay, either by working on a genre that is broad enough that it is unnecessary to decay, or that the executives feel that channel is good enough that they don’t need to. Of course obviously any channel that is not listed on this page would count as this, but we’ve listed a few (honorable) mentions.
- Influences On Other Media—Instances of this phenomenon when it comes to related mediums (Radio, Magazines, Websites, etc). Note that Magazine Decay has its own page.
- In Universe Examples -- In-Universe lampshades and mockeries of this phenomenon.
- Hey, we just mentioned how Syfy's decay went.
- See its example on the "Influences On Other Media" page, on the "Websites" tab, under "unique situations".