The original tax case came from the result of seven years of payments from Nokia India Private Limited to Nokia Corporation without any taxes withheld by the Indian company. As far fetched as it seems, the Tax Authority in India is claiming that Microsoft and Nokia revised the deal from an equity investment to an outright purchase of assets to help Nokia paying avoid Indian taxes. That does seem a little hard to swallow, but read the statement below and you can come to your own conclusion.
The Financial Express newspaper in India noted that "The ACIT (India's Tax Authority) has also drawn attention to the fact that the Microsoft-Nokia agreement has undergone two amendments without there being any independent valuation of assets by a third party."
source: FinanacialExpress via NokiaPowerUser, WMPoweruser