Fortunately, cooler lore-heads (Ziebart and Sacco) prevailed: they pointed out to me this comment by Maxim in the original post: "Zero lore. Maximum fun. HOORAY!" That perfectly sums up why Blizzard is doing this: there's no lore reason or explanation behind it, it's just a straight up fun thing for us to do to celebrate the five-year anniversary. Bornakk makes it official: Ony is still dead, and she's not coming back. This is a one-time thing on the 5th anniversary.
While, unfortunately, that may destroy some dreams about seeing similar old-world content come back to life (at least until the 10th anniversary, when we'll probably see Molten Core updated for the level 100 15-man raiding scheme -- drool), it means that there's no thinking involved about why Onyxia's back. She's back, we get to raid the original raid and pick up some great loot while doing it. Zero lore. Maximum fun.