When the news first broke yesterday about Onyxia returning to her status as a raid boss, I, probably like many of you, went into lore-generation overdrive. She's dead in the comic (Varian Wrynn decided her head didn't so much belong with her body any more), so how was she going to be a raid boss? I stabbed at ideas: she's in the Caverns of Time! Arthas could resurrect her as a Frostwyrm of some kind!
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.