Here's the breakdown:
* The Vogons: A race of bureaucratic, uncreative, and extremely unpleasant aliens are responsible for the destruction.
* The Hyperspace Bypass: The Vogons are building a hyperspace bypass, a shortcut through the galaxy, that happens to run directly through Earth's location.
* The Lack of Consultation: The Vogons don't bother to consult with Earth's inhabitants, let alone give them any warning. They simply give them a fifteen-minute notice before demolishing the planet.
This event is a major plot point in the first book, and it sets the stage for the entire series.
It's important to note that the "reason" for Earth's destruction is a bit of a joke. Adams uses it to satirize bureaucracy, the universe's indifference to humans, and the absurdity of a seemingly insignificant planet being destroyed for a grand cosmic project.
