Page 1055 — Butterfly Effect
I like this reasoning for Discord's escape a little (just a little) better than "the CMCs done did goofed again and doomed us all," for several reasons. Man, that's another vintage show/fandom thing: The CMCs as constant harbingers of accidental doom and destruction. That was their "bit" for the longest time. I like the new direction for them from Season 5 onward, though. It's late as I type this and I'm rambling.
Transcript:Twilight Sparkle: All of this just raises more questions… It clearly didn't matter that it was out in the open for 1000 years.
Princess Celestia: It was in storage for about a century and a half in the middle, actually.
Twilight Sparkle: What I mean is: Why has this thing – Discord – only escaped just now?
Princess Celestia: To put it simply… My sister and I are no longer the chosen bearers of the Elements of Harmony. You are.
Twilight Sparkle: What does that have to do with anything, though?
Princess Celestia: My sister and I were worthy once, in a bygone age. With their power, we cast a powerful enchantment on Discord. But, in essence, our wills were a key component of that spell. Now the Elements have bonded to a different group of ponies. The magic that used our bond, not yours, is weakening.
Rainbow Dash: You couldn't have given us a to-do list?
Princess Celestia: Until now, I didn't know about this… feature. The point is, in that environment… all Discord needed to revive his spirit was a little push.
Fluttershy: Was it the army of exotic animals storming a high-society party to find the leader of the criminal underworld who was also royalty? Was that the little push?
Princess Celestia: Well… I didn't want to point hooves…