When the iOS Golden Master (GM) release is sent to developers, Apple also opens up app submissions for the new OS. They also appear to segment out releases targeted for the new OS so that apps are available, though there is no solid evidence for this.
For example, iOS 8 was released on Sept 17th, 2014. On September 9, 2014, Apple opened submissions for apps targeting iOS 8:
Prepare your apps for the App Store by downloading the iOS 8 and Xcode
6 GM seeds now. With these latest seeds, Swift is now final and you
can submit your iOS apps written with Swift to the App Store. Learn
about iOS 8 for Developers.