Less baseball and more videogames
Nintendo has a long history in business diversification. From a company that has done it all with cards, vacuums, taxi, love hotels & more, it's not a surprise that it owned Seattles Mariners. After sailing off with the team, it's time to let it go.
Frozen jokes apart the signs of this sale were scattered in the latest years. During Iwata's presidency there was still a good commitment on the cause. Baseball is stil a big deal in the US and Japan, but transitioning to Kimishima as CEO, the idea of owning a major league team wasn't so appealing anymore.
Basically Nintendo sold a big chunk of its shares for $661 million, keeping only the 10% of the team. Howard Lincoln - legal/hero from NoA (Nintendo of America) - is no longer team's CEO, replaced by John Stanton, but he will remain in the board to actually oversee the remaining stake.
I am interrogating myself on why Nintendo decided to abandon this business. My first thought is that owning a baseball team does not represent the status quo anymore, I mean a lot of business man and companies invest in european football, for example, but baseball is not on the front page as it used to be.
The second reason is that Nintendo itself has lost interest in owning the team, Yamauchi really wanted the Seattle Mariners and he personally followed the team for quite a while, before giving total control to Howard Lincoln. Satoru Iwata had to keep the team since Yamauchi was still alive and japanes culture impose a certain respect to predecessors and seniority. Clearly Kimishima, older than Iwata, without Yamauchi's control, finally took the courage to make this decision.
The third reason is highly connected to Nintendo's primary business: videogames. It's no mystery that 3DS couldn't go near DS' success, and Wii U has not been a good console in terms of sales and revenues. So basically with a 3 year in a row loss and a possibly fourth coming after a kinda good 2015, Nintendo might want to gather some vulgar cash to be safer for future investments and to contain losses, especially thinking about NX release.