First of all, Nokia will be forking Android

Microsoft has earned more money from android (by lawsuits) then it earned from WP8. Maybe it is trying to earn some more by embracing and extending.

And another approach would be like, Samsung Crapwiz will look the same even if it switches to tizen or stays android. Similar to that, the latest Symbian phones already resemble androids both in UI and UX. So changing the underlying software would cause minimal attention. Nokia would only profit from this more mature and capable technology.