What causes battery to drain faster than it can charge?
I think you answered your own question ;)

Navigation uses a lot of power: it needs constant use of GPS along with reading the map and processing text to speech.  Throw in another power-intensive activity like bluetooth or the phone's speaker (or calling, or data, etc), and it's very conceivable that it would drain power faster than it can pull it from a charger, especially a car charger, though the exact specifics would vary from phone to phone.

Here is a great list of the things that use the most power on a phone.