If you haven’t used your passcode to unlock your iPhone or iPad for six days, an iOS 9 restriction disables Touch ID if you don’t unlock with a fingerprint within eight hours.
Great story from Glenn Fleishman about why we’ve been seeing unexpected passcode requests on iOS devices. It’s one of those things that was annoying, and that didn’t make sense.
Source: New Touch ID rules: Why you have to enter your passcode when you wake up | Macworld