So I'm not sure if others have come up with the same/similar theory but the idea won't leave my head.
SO! While we don't know why we will become a Warrior of Darkness till June/July lore wise, we do know its gonna happen whether you are a Dark Knight or not. We know from Temulun that a shadow is growing within us, one that doesn't seem to have to do with current events like she thought it did. But I say, that darkness has been there much longer then even she knew.
So back to HW! Remember Thordan? Saw the WoL as this dark figure before dieing? The Devs told us it was simply him seeing things before he died. But I say otherwise. It was this expansion that allowed us to use darkness for the first time as Dark Knights, this expansion where Elidibus started trying to get us killed (though also using us to help further his plans at the same time). His reasoning is that we are becoming too strong with Hydaelyn's gift and Light.
I instead say that it isn't our strength in Light that has Elidibus wanting the WoL dead, but our potential in Darkness. Thordan's seeing us in shadow? We were unknowingly acting as a Warrior of Darkness in that very fight, taking down a Light-sided Primal. Primals are bad no matter what, but it makes it easy for the Ascians to weaken Hydaelyn so they will use it and thus, they are usually Dark-aspected. But Thordan went against them and ultimately was a Light-aspected one, and our Light would be much less effective against him. Thus our unknowing (or slightly knowing for Dark Knights) becoming of Darkness to take him down.
It is this potential to become a Warrior of Darkness, one that not only Elidibus would have a harder time controlling but also the possibility of us using Light and Darkness in Balance that is leading him to try and take us down. All the worse for him that we will finally tap into the power knowingly come Shadowbringers.
As for how we would tap into the dark without really knowing if you weren't a Dark Knight? Thordan and his Merry Men killing of Haurchefant, combined with the events leading up to HW in general, lead to the WoL being more likely to go after Thordan with darker emotions then our usually reasons for stopping Primals and Ascians. Even if your WoL was able to keep a more level head it still simmered in the back of their mind, allowing them to tap into their potential with Darkness.
And thats my theory! We already are technically a Warrior of Darkness, ShadowBringers will just let us tap into it on the fly like we do with Light. More so for Dark Knights.