This is such a delightful word I have caught myself using, and is particularly noteworthy in its conventionality of etymology. It seems to me (and others) that its slow introduction to the English language is due to the Spanish word for "storm," tormenta. Slowly -- possibly through the American/Mexican border, the word has begun to take on an English meaning of "stormlike, thunderous, violent, fierce," or something of the like. In a hundred years, Webster will surely have it listed, with a date near 2000, and show the etymology is indeed Spanish.
A Google search for "tormentuous" reveals far too many results to be merely the result of freak chance, and all uses tend to have the same meaning. And this is no neologism contrived for an express purpose by a single person or group. Instead, we are witnessing the slow growth (1,040 Google hits) of a new word growing by traditional methods of linguistic transformation.
Think of this memetically: Why has it taken off so well? It must be, as a word, good for its own survival, in out-competing others like it for the same "resources." In this case, the word mimicks the sound of "torrential," a word already established and well associated with storm-like traits, as well as "tempestuous," which invokes rage -- particularly in nature. By inserting the Spanish word "tormenta," we have an absolutely plausible English word that could mean exactly "storm-like," but which benefits from the quasi-homophones' meanings of "violent amounts (of water)" and "windstormy and angrily emotional." Then there's "tumultuous," which implies disorder. Finally, there's the English "torment," which implies torture. When I think of "tormentuous," I tend to combine all four: an emotional/angry disordered paroxysm of emotional outlash, a deluge of psychological anguish, destroying everything in its path. This could be metaphorical.
Because of these homophones, it's possible that "tormentuous" is a particularly common mutation in human minds, and one that doesn't tend to be scoffed at or lead to confusion -- it doesn't have any inherent survival disadvantages. In this case, we could be seeing many separate mutation events happening (as well as standard reproduction), and subsequently we will see the populations combine and grow by mutually supporting themselves.
What a fantastic confluence! Because of it, we have an opportunity to watch this word -- which has tremendous selective advantages -- really take off.