Environment
Answer
With Android 5.1, Google implemented a kill switch that is automatically enabled when you set a screen lock and link the device to your Google account. A Google account username and password are required to complete the setup process after a frozen factory reset. Without the necessary login, the device is essentially “frozen” for anyone but the rightful owner.
Additional Notes
To avoid this, complete a factory reset from the settings app on your phone prior to selling, returning or giving the phone away.