Microsoft's Steve Ballmer shocked the world when he announced earlier this year that he planned to retire, and it left many wondering what exactly happened to prompt this decision. Well, thanks to a candid interview with the chief executive, we now know.

Speaking to The Wall Street Journal, Ballmer admitted he's an "emblem of an old era" and has to move on. The 57-year-old added: "The best way for Microsoft to enter a new era is a new leader who will accelerate change."

But that's not the only reason why he's leaving. It seems Microsoft's board of directors had been pushing Ballmer since January to quickly transform the company into a firm that focuses more on devices and services. He singled out lead director John Thompson, claiming Thompson told him: "Hey, dude, let's get on with it. We're in suspended animation."

Thompson told The Wall Street Journal that Microsoft's board didn't push Ballmer to step down, but the directors did push him "damn hard to go faster". Ballmer apparently acknowledged to Thompson that he could move faster. However, in May, the CEO questioned whether he could perform fast enough to satisfy the board.

It was then that Ballmer realised for the first time that Microsoft could change faster without him: "At the end of the day, we need to break a pattern. Face it: I'm a pattern," Ballmer said.

READ: Microsoft announces CEO Steve Ballmer to retire

So after originally saying in 2008 that he intended to remain CEO for another decade, Ballmer announced his retirement in August.  The reason then was supposedly based on the loss of billions in acquisitions and the Surface tablet. Ballmer said he would give up the reins following a 12-month transition.

It seems the chief executive - notorious for showing his excitement and wide range of emotions during interviews and keynotes - wants to set the record straight, as well as sacrifice himself to keep Microsoft going strong for years to come.