Microsoft has been almost completely shut out of the mobile phone market by companies that actually know how to write software. Due to their absurd cash reserves (resulting from their near-monopoly of PC operating systems), they are able to use the monkeys on typewriters approach, entering a market repeatedly until they happen to hit the right note with a product.
Microsoft is about to reenter the mobile phone market and to increase their chance of success, they’ve decided to preemptively sue Motorola for daring to make phones with the superior Android OS. This will likely fail, but at this point Microsoft still has lots of money and lots of lawyers on retainer. They’re already despised by most intelligent consumers, so they must figure they have nothing to lose by attacking other companies based on ridiculous patent claims.