Looking at NBC's new autumn schedule, the key point of note is that Constantine won't be getting on air until October 24th, the same night that Grimm will come back.
There's no reason given, and pretty much everything else is rolling out in September 'as normal.'
I guess they're looking to avoid long breaks in service? Maybe it's a scheduling thing with some of the talent they want for the episodes?
Anyway, it's close to Hallowe'en so it doesn't seem entirely inappropriate.