Yudda if the device isn't rebooting, then I believe the only time the pin is required (instead of fingerprint) is if it's been >48 hrs (?) since you last entered your pin or you've failed the fingerprint 5 times in a row.
Are you sure you're not accidentally bumping the sensor in your pocket or something? I could have sworn there was a "pocket detection" setting somewhere but I'm not seeing it.