The question is simple, but the answer seems anything but.
If we are to take the tales of Trinimac's downfall at the hands of Boethiah as true, with the former being transformed into the Daedric Lord Malacath as a result, it raises the question (at least for me), why did Boethiah chose Trinimac specifically to make undone?
If it was possible for Boethiah to debilitate and/or imitate Trinimac, supposedly one of the mightiest Aedra, so completely, why did Boethiah not set his/her sights a bit higher and attempt to bring down the head honcho, Auriel/Akatosh, himself? What made Trinimac so susceptible to sabotage? Was Trinimac the only Aedra Boethiah could have brought down? If so, what about him made that possible?
I might suggest that, with fables surrounding the event of his fall, Trinimac may have grown complacent with his place in the cosmos or perhaps Boethiah thought he commanded more deference and reverence than he deserved, intervening too often and too directly in the affairs of the Aldmer, telling them how to view their place in the world and such. But, seeing as Malacath is a being who champions the spurned and ostracised and is fixated on the following a strict code of honour and ethics, I find it hard to believe that Trinimac was a being prone to hubris.
One thing that strikes me as particularly interesting and curious about the whole affair is the stories surrounding the rise of the Chimer. It is said that Boethiah claimed that Trinimac and presumably the other Aedra were wrong to tell the Aldmer that they should despair over their mortality and the existence that Lorkhan had bound them to. Was Boethiah's protest based on some characteristically atypical sense of justice? Or perhaps it was a vendetta brought on by some sense of fraternal duty to another divine Trickster? Or maybe (s)he just thought making accusations like that were a sure means to plant the seeds of chaos and confusion in Aldmer society?
Another line of questioning is, why did Boethiah bother with any of it? Was it just the anarchistic Prince's nature compelling him/her to screw with the Aedra and turn their followers against them or were Boethiah's motivations deeper than that? Did the Lord of Upheaval covet devotees? That doesn't strike me as his/her MO... Just what the heck was it all about?!