Macworld:

A previously undocumented requirement asks for a passcode in a very particular set of circumstances: When the iPhone or iPad hasn’t been unlocked with its passcode in the previous six days, and Touch ID hasn’t been used to unlock it within the last eight hours. It’s a rolling timeout, so each time Touch ID unlocks a device, a new eight-hour timer starts to tick down until the passcode is required. If you wondered why you were being seemingly randomly prompted for your passcode (or more complicated password), this is likely the reason.

Good to see Fleishman getting to the bottom of this but very annoying that Apple instituted this new procedure without letting the user know. Many of us have been confused and/or annoyed by this for months.