Currently, details are sparse, there's a lot of "We don't know" in this statement, and that's fine. It's still early days.
In short, Notch wanted to focus less on running a massive company and decided to sell. Mojang and Microsoft have a good relationship, and it seemed like a good fit for continuing Minecraft's development.
I'll say it seems like a great deal for Microsoft too. $2.5 billion seems pretty cheap for the game every 7 and 8 year old can't stop talking about.