I'm not in the industry, or Rian, or even someone who knows how these things go, but I'll take an educated guess that it's part of the studio's release schedule. The summer movies are all plotted out ages in advance, different studios "get" Memorial Day weekend, or 4th of July, or the corresponding bank holiday weekend if you're not in the US, and while they're not exclusive, it's rare that two studios will try to drop their quarter-billion-dollar gambles on the same weekend as someone else is doing the same at the risk of cannibalizing the all-important opening weekend haul.
What I'm saying is that it's easy for a smaller movie to get completely blown out of the water by the three months of blockbusters that summer brings, and so the studio has placed the release after the rush. I don't know if September is particularly good or bad in this case (versus, say, April), but it certainly is better than dumping it in the graveyard that is January-release movies.
For the production schedule at this point, I'm only guessing off Rian's Tumblr and various cryptic posts here and there, I think the editing process wrapped up a few months back, the score is either in the works or complete by now, and they're now probably doing visual effects, and the whole thing should be in the can by the end of the year, if not sooner.
As for what Rian will be up to in the intervening time, I bet it involves an opium den. But more likely is writing something new.