In Tom DeFalco's awesome interview book, Comic Creators on X-Men, Scott Lobdell recounts the state of the X-Men books after the bold title-wide Age of Apocalypse storyline.
Lobdell mentioned that after the crossover ended, each title was basically given free rein to try out any type of story they wanted. Warren Ellis wanted an issue of Excalibur where they just went to a pub - he got to do the story. Well, Lobdell's idea was for some bad guy to toss Juggernaut through the sky, and all Juggernaut would be able to tell the X-Men was one word, "Onslaught."
All the other writers were intrigued, but when they demanded to know who Onslaught was, here is what Lobdell said:
I told them that I had no idea, but I just thought it was a cool way to open a story. Imagine someone so strong that they could hurl Juggernaut across the sky! I ended up doing that opening sequence, but I still didn't know who Onslaught was.
That became a problem later on, when other writers were told to give hints to Onslaught in their titles, but didn't know who Onslaught WAS!
Larry Hama's clues in Wolverine, in particular, really didn't jibe with the later revelation that Onslaught was Professor X himself, corrupted by Magneto's mind.
But luckily for Lobdell, when Marvel needed a big threat to lead into Heroes Reborn, he happened to have a mysterious big threat in his back pocket, and when it was determined that it was an evil Professor X, then suddenly Juggernaut (Professor X's step-brother) being the first victim made a whole lot more sense.
Alls well that ends well! Thanks to Tom DeFalco for the interview and Scott Lobdell for the candor!