Has it been revealed exactly why Sylvanas is ripping a hole in reality to go to the Shadowlands?
Not entirely. There's various cinematics and story bits you could watch, but what it boils down to a couple possibilities:
One is she's convinced there is some ultimate evil behind everything (which is probably Void Lords and loosely... probably true), and that the only way to win is to fight the ultimate enemy, but life and things like 'hope' are obstacles to that.* So apparently she wants everyone to die and empower her to go get the baddie. And to do that she's going after some ancient evil (that's never been mentioned before) imprisoned in the Maw zone of the Shadowlands and something something profit.
*seriously, the hope thing is a weird trigger. The original war plan at the beginning of BFA was to kill Malfurion and take Teldrassil, yielding thousands of hostages, basically securing the entire continent as a Horde stronghold. Then a random sentinel captain told her that she can't 'kill hope' and she burned the world tree to the ground in a fit of rage and/or 'Oh, yeah? Hold my beer.'
Second possibility is this is all pure crap, and she's just a nihilistic monster who wants to kill everyone for the lulz. The 'storytelling' is that vague.
Alternately, (and this is more in line with her actual behavior and old storytelling- see the story link in the first post) she's selfish, only cares about herself, and sees other people as "arrows in her quiver." And she made a series of deals, first with a cabal of Valkyr (who've been following her around since Cataclysm, and dying one by one in various incidents, and once the last one dies, she's toast), then Helya, and lately Azshara herself, all the while scheming to come out ahead. This isn't entirely incompatible with the second interpretation, that she wants to kill everyone and come out ahead, but Blizzard assured everyone at BFA's launch that the story was complex and morally grey, which should mean she's not an evil person of evilness that just wants to kill everyone evilly. But that promise has honestly never materialized in game, and I have doubts there is any meat to it.
It's an even toss up if Blizz is going to keep her actual personality and behavior intact, go full murderous rampage for no raisin, or go for some sort of crazy redemption arc like they have repeatedly used for other characters in various franchises (see LordofHats' post above)
As of the current patch (8.25) she's abandoned the Horde with a vague promise of making everyone see and that they are all nothing, and her followup is apparently to make this gateway.
Mind you, the finale of BFA hasn't happened yet, so it isn't clear if she has another move in patch 8.3 (which is partially up on the public test realm), for the big confrontation with the last Old One (on Azeroth), N'zoth.
The reveal of the new expansion rather pre-empted the finale of the current one.
And more than likely there will be a Christie Golden tie-in novel in spring/early summer that will come up with a completely different explanation for literally everything. Or not address it at all...