The tail wagging the dog describes a situation in which the thing which should be driven by circumstances instead is the thing driving the circumstances, it is a role-reversal. In modern times, an example would be the phenomenon of a news outlet reporting about an unimportant situation so often and so fervently that the situation becomes important, instead of the news outlet scheduling its reportage around situations that are inherently important. The idiom the tail wagging the dog seems to have originated in the United States in the 1870s. According to Google Ngram, the popularity of the idiom tail wagging the dog has risen steadily since then. A popular movie about political manipulation of the news media debuted in 1997, Wag the Dog.
Examples
The tail is wagging the dog: Gluskin Sheff’s George Young explains the 2016 market turmoil (The Financial Post)
Otherwise, the parents will quickly lose control over the child and there will be a power shift, similar to the tail wagging the dog. (Nevada Business)
But it is the tail wagging the dog; it is a clear example of why many people have become disillusioned in their political representatives. (Forbes Magazine)
“To go back and just wholesale clear all these terms out of the long-existing code to deal with these marginal cases, I think, is just the tail wagging the dog,” he said. (The Deseret News)
From the beginning of the Bergdahl/Taliban saga, I’ve suspected the Obama administration was wagging the dog to get the VA scandal out of the headlines … (The Iowa Republican)
“It’s the tail wagging the dog very much here in Donegal when it comes to fixtures and playing of club games,” blasted Ardara senior team manager Adrian Brennan, when contacted on the matter by the Democrat. (The Donegal Democrat)