A Print Media Trope. Ever notice that whenever the papers or the mags have a particularly spurious rumor about a person, TV show, upcoming movie the source of this rumor is never named?
They are usually credited as "our source", "an insider", or, in the case of Celebrity Gossip, "a friend of X". Why is this person never named? It could be because these sources' jobs or friendships could be in jeopardy if they are named, or they could open themselves to libel suits or worse...
...Or, with the sillier rumors, they've either been made up, or they are an "<Insert Network Here> Insider" insofar as the "insider" is a coffee boy or some celebrity's hairdresser or something.
The trope name comes from Outpost Gallifrey Doctor Who Forums, where, due to the above quote, Alan Fridge was a Running Gag in general, but especially when it came to the daft Doctor Who rumors that the British tabloids are known to print. It should be noted that it was a different poster who came up with the name Alan Fridge, but it was Steven Moffat who linked the name to the trope.
Also applies to some stories in newspapers on other subjects, e.g. defense. In the UK, stories about the private lives of the Royal Family inevitably come from "a palace insider".