Of course you know why we do it. In a classic example of metonymy, the media and culture at large began referring to the 1972 political scandal that eventually forced Richard Nixon to resign as "Watergate," the name of the hotel and office complex where burglars tied to the administration were caught trying to break in to the Democratic National Committee headquarters.
But how is it that, over 40 years later, the NFL can find itself mired in a "deflategate"? When did that syllable become such an accepted suffix?
Pretty quickly, it seems. News of the original Watergate burglary and coverup broke in June, 1972. During the following year, newspapers might have described sketchy behavior as "Watergatery" or an unscrupulous politician as a "Watergater," evidence that the scandal had instantly permeated the language. Almost a year after the actual Watergate, the first example of the -gate suffix appeared in an August 1973 issue of National Lampoon magazine as part of a satirical story about a fake Russian scandal, which the magazine dubbed "Volgagate." That story made a direct comparison to Nixon and the actual Watergate, so the usage is more about drawing a parallel within the piece itself.
William Safire, a former speechwriter for Nixon, is often credited with forcing -gate into the popular lexicon during his career as a New York Times columnist. In a 1996 issue of New York magazine, Noam Cohen takes aim at Safire, compiling an "abridged dictionary" of -gates coined by the columnist and accusing him of an attempt at "rehabilitating Nixon by relentlessly tarring his successors with the same rhetorical brush—diminished guilt by association."
(Safire later admitted that this accusation might not be totally off-base in Eric Alterman's Sound and the Fury: The Making of the Punditocrcy. Alterman writes: "Safire today admits that, psychologically, he may have been seeking to minimize the importance of the crimes committed by his former boss with this silliness.")
Of course by then, Safire, or anyone else, didn't need an excuse to use to the suffix. In 1993 the scandal-based definition was added to the Merriam Webster dictionary.
[h/t for the idea to the fine folks at the John Carney show on KTRS]