It's very likely that the interactive doodle celebrates Google's 12th birthday. Google usually celebrates its birthday on September 7th or September 27th, depending on the year. "Google opened its doors in September 1998. The exact date when we celebrate our birthday has moved around over the years, depending on when people feel like having cake."
Happy Birthday, Google!
Update (via Blogoscoped Forum): Google says that the animated doodle didn't celebrate an important event. It was fast, fun and interactive, just like Google.