Uh, what?

You read what I said. Microsoft used to
completely own many segments of the markets. We're talking 99% market penetration. Nobody thought they could be beaten, and everybody believed that even if they ever felt threatened, they'd either buy the competitor out or out-develop them by throwing a thousand developers at the problem.
But despite their dominance, they're being challenged and pushed back basically across the board:
On the O/S front while they are still fairly the big man Linux has made tremendous inroads and now dominates the server operating system market, Apple has an increasing share of the O/S market (and, some would argue, a technically superior O/S).
On the office suit front they are facing competition from OpenOffice on Windows, LibreOffice on Linux (where they don't compete) and Apple's suite on OSX. Even Google Docs is giving them a run for their money on the desktop, and it's a web-based cloud-hosted suite. Of course, Google Docs completely demolishes Office 365 when it comes to cloud-based offerings.
On the database front, they are facing a ridiculous amount of competition by Oracle (as Oracle and as mySQL), and a host of open-source databases, including upstart MariaDB.
On the browser market, IE, which used to utterly dominate the market, is being destroyed by Chrome, Firefox and Safari, and to a lesser extent Opera.
On the cloud services front - both storage and compute - they are vastly outmatched by Amazon and Linux. The Azure offerings don't even compete.
On the virtualization front they are playing catchup to VMWare (aka EMC) and their Hyper-V offerings still don't compete favorably.
On the mobile O/S front they are being completely destroyed by Apple and Google and despite pouring billions of dollars of money to develop and market Windows Mobile and to lure developers, they are getting no traction (except the traction they bought in Nokia Oyj).
That's not to say that they're done for. Far from it. They're fighting and fighting back hard. But the aura of invincibility they had is gone and everyone is smelling blood.