A 5.44 billion EURO transaction to acquire a company that is already in a strategic partnership with you and working exclusively with your platform doesn't make sense.
My vote also goes for Nokia being financially in trouble, and M$ have their hand forced by the fear of a collapse in Nokia would gut the distribution channels for all the flagship windows phone models.
Worse, what if word got out and someone else bought them?
...wait, no, that would mean Microsoft would have some special insider knowledge of Nokia's finances that no one else was party to. Surely not.