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