A ‘landslide’ is usually associated with something bad. If that is the case, why is it that during elections, a big victory is called a ‘landslide victory’? 

Landslides are a common occurrence during the rainy season in hilly and mountainous regions. Heavy or continuous rain usually causes mud and rocks to come sliding down the mountain/hill, and in the process, destroy everything along their path. Houses, vehicles, people, trees and animals often get buried in a landslide. While it is true that a ‘landslide’ is usually associated with something bad, a ‘landslide victory’ is seen as something good — at least from the point of view of the winner!
When someone has a landslide victory in an election, he wins the contest by an overwhelming margin. He defeats his rivals soundly. What is it that a landslide does? It buries everything along its path. Someone who wins an election by a landslide, destroys everything in his path. In this case, it suggests that the candidate doesn’t just beat his opponents, he buries them! 
The term became popular in the 1800s to describe a victory in which the opposition is "buried", similar to the way in which a geological landslide buries whatever is in its path.

The young candidate won by a landslide.

The local MLA believes that his party will register a landslide victory.