Why would blood pressure decrease when you have a ruptured spleen?

Bleeding: Rupture will cause bleeding into the peritoneum resulting in hypovolemic and possible septic shock.

Loss of Blood: Any time one looses significant amount of blood it can cause a fall in blood pressure as blood volume goes down. Ruptured spleen always cause significant loss of blood and that is the cause of abrupt fall of blood pressure.

Hemorrhagic Shock: In severe splenic trauma with associated bleeding, hypotension (low blood pressure) is likely due to blood loss.