Why do fish jump out of the aquarium ?

Many times, aquarium lovers are shocked to see their pet fish lying dead on the floor near the aquarium.  This can be prevented by having a proper cover or a hood over the aquarium tank.
In the aquarium, fish jump due to the following reasons

Poor Water Quality.
Ensure that the water in the tank is periodically changed and that the tank has an efficient filter system. 
Poor oxygenation
Adequate oxygen levels need to be maintained by means of proper aeration. 
Chased by bully fish.
Fish can jump in order to escape from a bully fish.  Ensure that all fish are of a peaceful temperament.  See Article Handling Bully Fish

Jumping out of the tank is one of the many dangers for aquarium fish.  In the wild, fish jump to catch foods such as insects resting on overhanging branches.

Fish also jump to move to a nearby water body if the water they are living in becomes inhospitable due to low oxygen content or dries up.