As of now, there's no info on why Foster left the company. Google acquired him just seven months ago from e-commerce giant Amazon to strengthen its position in the hardware department and immediately after he roped in, Google announced the Pixel phones.
While in this stint with Amazon, Foster and his team developed various hardware products from including the Amazon Echo. It is also reported that Google is working on three Pixel phones for 2017, and the Foster is also leading the team, but his sudden exit turns down some shackles.
Foster, however, did not make any public statement on his next journey. We're expecting him to join Andy Rubin's Essential, but not sure about the same though.