So the CEO of Microsoft is what they call in politics a "lame duck" leader -- someone whose authority is weakened by the knowledge that he won't be around much longer. Ballmer’s ability to rally and unite Microsoft’s waring divisions has been seriously compromised.
Just as in boxing, where you really go after your opponent when he's winded or injured, Apple is seizing the opportunity to kick Microsoft while it's down for maximum effect.
And second, with the decision to directly sell Surface tablets and the more recent decision to acquire Nokia, Microsoft has for the first time ever actually entered Apple's business directly.
Apple and Microsoft used to be “frenemies” engaged in “coopetition,” both competing and partnering, and each company offering software on the others’ platforms (in Apple’s case iTunes and Safari).
Microsoft used to be a software company selling software to OEM partners and consumers, who actually bought their hardware from other companies. But the old PC business is in steep decline, thanks especially to consumer embrace of tablets like the Apple iPad. Microsoft's response to this is to augment the old Microsoft model with a new Apple model of selling integrated hardware, software, services and content.
While Google appears to be the major competitor, in fact Microsoft is now a more direct competitor.
For example, Google's massive market share leadership in smartphone operating system mostly doesn't benefit Google or take sales away from Apple. The majority of Android deployments are for third-world, no-name, zero-margin devices that don't come with access to Google's Play store. These users aren't "customers" of either Google or Apple, and really aren't relevant users in Apple's mind.
But buyers of Surface tablets or Nokia phones are, in fact, the exact same customers Apple is going after. Every sale of a Microsoft device is a lost sale for Apple -- and a lost opportunity for future software, service and content sales.
And that's why Apple is now gunning for Microsoft.