Do big events typically originate with a writer or an editor? I'm sure at least some of them come from a writer (like how Axis grew out of an Uncanny Avengers arc), but I also imagine that if events weren't coming up organically, editorial would try to get them in there somehow.

It’s kind of a mix of everything, as most Events become Events at our Editorial Retreats, which include our major creators. Through the course of discussion, certain ideas become apparent to the people in the room that what is being spoken about is an Event, and so it becomes one. Most of those ideas originate with a writer, but through the course of the discussion, all sorts of other people may have tossed elements into the mix.