The revelations in the Future arc episode 11 make it weirder that Asahina had a faked death. You can't assume that Gozu did it or Asahina did it anymore. The only explanations are:
It was something that wasn't thought out well and 'just happened.' Pretty unlikely, I think.
There's the mysterious 16th person who decided to have a little fun with things for the sake of messing with them. Also pretty unlikely since they sealed themselves in that room and I don't recall that haven't changed by the time they awoke.
Aoi or Ryota (And maybe Naegi too but let's ignore him.) did it for the same inconsequential reasons. Suggesting that Aoi is evil probably doesn't sit right with people. I sure don't like it particularly, but it's possible I suppose.
Then there's Ryota. A much more likely candidate, given that he helped Junko with the brainwashing videos before and was recruited by Kazuo Tengan, who was intimately aware of everything going on.
There's still the question of 'why,' but Episode 11 narrowed the 'who' down to just a few people.