I just caught this on Last Call with Carson Daly. He did an interview with him - James Gulliver Hancock.
http://jamesgulliverhancock.com/
http://jamesgulliverhancock.com/biography/
http://allthebuildingsinnewyork.com/new/
He said it started out when he was on a trip to London and it was raining a lot. So he started to draw rain drops and ended up selling the print. He then drew cars in LA since he said everything seemed to revolve around cars. And then on a trip to Paris he drew rooftops, and that's when he said he noticed the architecture and all the detail.