Why is the spleen prone to rupturing?

Soft on the inside: Some parts of the body are soft on the inside and can't take a direct hit, or will rupture into pieces. Soft on the inside means not having tough fibrous strands to hold things in place. The brain is soft on the inside, but the skull protects it. The spleen is normally protected by the rib cage. In some diseases, the spleen enlarges and sticks down below the rib cage. Hit hard enough, it ruptures.

Splenic rupture: Location as well as it filters lots of cells through the blood, so it tends to get bigger with time. With significant trauma rupture is possible, but it does not happen with everyday trips and falls.