I'm a flight medic, no doc! Spleens are commonly ruptured simply because of the force involved in the accidents. I don't wanna get too technical...here goes.
Deceleration injuries (it's not the fall, it's the sudden stop at the bottom that kills you) cause the organ (ie: the spleen) to get shaken around, banging into to the ribs (which may/may not be broken), and causing cuts or lacerations in the organ itself. You can also damage the ligaments holding the organ in place. The forces involved in a bad get-off can also squish the spleen to the point where it bursts (think of slowly stepping on a tomato).
The location of the spleen (upper left quadrant of your belly, just under the ribs) makes it a bit of an easy target (so to speak anyway).
Luckily, you can live w/o your spleen. Hope this answers your question.