Sometimes if the driver opens the app too fast before it gets a GPS/location lock, it could result in address unknown. Or if Google Address Lookup API doesn't find an address for that specific geo-location.
Or a much less likely case, if driver erased address and typed "Unknown address..."
Basically, the address field in the app is a regular editable field just like Description. We try our best (most times it works) to pre-fill it as a helping feature... but if the address isn't automatically obtained, for whatever reason mentioned above, the driver's should consider putting the address in the address field before submitting the lead.