Not totally sure but this is what happened to my GSD (she was 5).
One day I had gone to work as normal at 8 oclock, gave my dogs a handful of cooked liver before I left. Came home at dinnertime 12 ish. and let my dogs out. Sheba was sick in the back yard, I could clearly see pieces of the liver still whole. Took her to the vets that afternoon. The vet examined her and said that she thought the spleen felt enlarged, I could not feel anything unusual. Took her back two days later, another vet examined her, said he could feel something enlarged, but was not sure whether it was the spleen or the liver. Booked her in for an operation (this was Friday) on the Monday, said he was glad he was not operating!
The vet operated on Monday, said that dogs can survive well without a spleen, the operation should take about an hour at the outside. Two hours later she phoned me, said the spleen was massive and had twisted (torsion), she said it weighed about three times what it should, it was full of blood, but the good news was that it was smooth so there was no cancer.
My bitch recovered well, but sadly I lost her 6 months later with bone cancer in her hind leg.
My understanding is that the spleen helps the immune system, and I wonder to this day if removing it helped the cancer to spread more rapidly. I also wonder if the spleen being enlarged was a sign that it was overloaded trying to fight the cancer.
I think that the spleen helps to make and store the blood cells so that could be why it can give rise to massive internal bleeding.
The spleen, both in humans and animals, is still not totally understood, we are still not totally sure why it is there, or what it is used for.