Well, this is a surprise.

Erich Ringewald, co-founder and former CTO of promising international mobile payments startup BOKU has quit the company to join American Express as VP of Software Product Management for the U.S. Consumer Services business.

At AmEx, Ringewald will help design software-based consumer products.

It seems to be a rather sudden decision, considering Ringewald’s LinkedIn profile hasn’t yet been updated, nor has the ‘Management’ section on BOKU’s website.

It’s a strange move for Ringewald to make either way. Formerly a General Manager and VP of Amazon.com, where he led the development of AmazonMP3, Ringewald co-founded BOKU in 2009, but it was far from his first entrepreneurial venture.

Way further back in 1990, he co-founded the legendary PC operating system company Be Inc. with another Apple refugee, Jean-Louis Gassée.

BOKU’s global payments platform now operates in 66 countries with 245 wireless carriers, and the company has raised over $70 million to date.

From the outside at least, it looks like BOKU is just getting started, so the question remains: why did Ringewald leave now?

We’ve reached out to BOKU and Ringewald to learn more.