Andy Rubin, the man who co-founded Android, Inc. before it was acquired by Google and fueled the mobile platform into popularity, is leaving Google.

This is after Google CEO Larry Page announced last week that current Android head and chief of Chrome and Google Apps Sundar Pichai will be taking over most of Page's day-to-day responsibilities. The Google chief himself is focusing on the wider strategy of having Google behave once more like a hungry startup rather than an aging corporation.

Rubin, who left Google's Android team earlier this year, is currently in charge of the nascent robotics team and has overseen the acquisition of a number of robotics startups, such as Boston Dynamics, which is known for its humanoid robot Atlas. Google says Rubin is leaving to pursue his passion for successful startups by creating an incubator for young companies building technological hardware products.

"I wish to Andy all the best with what's next," says Page. "With Android he created something truly remarkable -- with a billion-plus happy users. Thank you."

Earlier this week, Google announced Pichai, Rubin's successor as head of Android, will be taking over six more divisions at Google, including search, research, maps, Google+, commerce and infrastructure. That is already on top of Pichai's responsibilities of overseeing Android, Chrome and Google Apps. Rubin's own robotics team falls under research, effectively handing over to Pichai for the second time what was once Rubin's domain.

Google declined to provide details as to why Rubin is leaving, but a person familiar with Google and Rubin tells The Wall Street Journal that "Rubin is an entrepreneurial spirit who likes to run his own show and was facing constraints on his activities at Google." As former Android chief at Google, Rubin has long been known to keep his team closely guarded from the rest of the Google team. For a time, Rubin's Android team was said to have its own lunchroom. Pichai, on the other hand, is said to be more collaborative and diplomatic, traits that are more useful in keeping the internal conflicts in check within Google as growing teams start to see overlapping responsibilities.

Rubin's departure will deal a blow to Google's burgeoning robotics efforts, and some analysts are surprised by the "unplanned" leaving.

"If it was voluntary on Mr. Rubin's part, you would think he would see part of the robotics project through to completion to have something to show publicly before leaving," says analyst Scott Strawn of IDC.

James Kuffner, who worked on Google's self-driving car project and spent seven years working on humanoid robot technology at Carnegie Mellon University, will take over Rubin's responsibilities as head of the robotics team.