<conspiracy theory> Apple releases latest version of iOS (which requires everyone who installs it to type in their passphrase, for both the phone and iCloud, again), and the FBI backs off on demanding that Apple write FBiOS for them.

Maybe Apple got hit with an NSL and were actually required to backdoor their products? Now Apple is delivering the full encryption keys to interested parties? </conspiracy theory>

I know it's silly, but the coincidence of the release of iOS 9.3 and the FBI backing off from its position was striking.

The more reasonable theory is, of course, that the FBI realized that they overstepped their boundaries and put a tool they've used for decades ex parte at risk of being taken away from them by butting heads with a determined adversary.