The problems reportedly started around 2001 when a power struggle arose, Ballmer was the new CEO at the time but Gates was still around, which created complications because Microsoft employees didn't knew who was in charge. Ballmer explains that among other things, when an engineer was displeased with him, he'd go to Gates, and notes that he didn't really feel in charge until Gates left entirely in 2008.
Ballmer blames Gates for wasting the A-team resources on Longhorn from 2001 to 2004, an ambitious operating system project that didn't pan out as planned. Microsoft had to restart from scratch three years into it and eventually released the project as Windows Vista, which lacked key failings and had many failings that enraged customers. Looking back, Ballmer notes that these resources would have been much better spend on phones and browsers. Despite being CEO at the time, Ballmer claims Gates is ultimately responsible for the disaster that became Windows Vista.
Another major factor was Ballmer's idea to purchase Nokia. When he pitched the acquisition to the board in June 2013, they didn't approve and Ballmer was reportedly even more upset because Gates didn't support his pitch. Ballmer drew the line in the sand and said he'd leave Microsoft if the Nokia deal didn't go through, which resulted in the board accepting an altered version of the deal. Ballmer resigned in 2014 and bought the Los Angeles Clippers basketball team for $2 billion.
Ballmer drew a line in the sand, and said he’d leave if the Nokia deal wasn’t accepted. The board caved to an altered version of the deal — likely to take Nokia mobile, not the entire kit and kaboodle — to appease their CEO. Ballmer resigned in August.
Though Ballmer admits Microsoft missed the mobile train under his stewardship, he points to Gates’ Windows Vista as the problem. According to Ballmer, they had all their talent focussed there instead of other areas. Codenamed Longhorn, Vista also took up six years of Microsoft’s time, with Ballmer saying “I put the A-team resources on Longhorn, not on phones or browsers. All of our resources were tied up on the wrong thing.”