Many cable channels are created to fulfill a specific programming niche. The Golf Channel shows golf, The History Channel shows history programs, and so on. Some channels, however, are not as wedded to their original concept as others. 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- to 31-year-old males, and Professional Wrestling is hot among that demographic, surely no one would mind if we started showing Professional 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.
Part of the cause seems to be the fact that the channel is originally created to air shows that are "in the vault" of the company that creates the channels, but soon, the channel's own executives discover that original programming nets them more money, and the new stuff slowly displaces the old. Reality Shows, as a genre where the cost to produce is especially low, are common here. This may result in a new "vault" channel, which slowly undergoes the same process. (Nicktoons Network from Nickelodeon, for example.)
Note that one way to tell if this is happening is if the name of the network is hidden behind an acronym. For example, The Nashville Network referred to itself more and more as TNN (it eventually even changed what it stood for to "The National Network") before becoming Spike TV; similarly, you'd hardly know that TLC was ever called The Learning Channel.
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. But mostly it's just good old-fashioned selling out for ratings — whether it works or not. Competition with new media is prevalent as well, as classic reruns give way to DVD box sets, music video channels give way to You Tube 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.
Otherwise known as Viacom Syndrome.