I am really sorry to hear this about Jake.
Going by the symptoms you describe, his age and breed, I would think the most likely reason for his death is what is know as Gastric Dilation and Volvulus.. This basically a bloat with a stomach twist. For some reason, the stomach fills with a little gas - sometimes becuase they eat their food too fast, sometimes for no reason we can see. In deep chested breeds such as GSD's and other like Dobermann's, the stomach is able to 'float' upwards in the abdomen because of the gas, and eventually it can actually twist over. The initial phase of the bloat can cause some abdominal discomfort, sometimes retching and nausea (a common reason why dogs eat grass is nausea). Once the stomach has twisted, this becomes an acute emergency as it constricts the blood supply of the stomach, and becuase of the anatomy, the spleen. Once this blood supply is constricted, the dog can enter a shock state very quickly, and die very quickly.
would the sudden soil-eating be his attempt to 'clear himself out'?, there was no grass in his mouth just soil
It is a possibility. The nausea can cause them to eat odd things, and also they can be a little distressed and may be the act of digging was helping his stomach feel better.
He was always a very 'nervy' dog, hyper alert
Not uncommon in a guarding breed like this. It is a horrible thing to have happen, and always so sudden as well.