You might be drifting into the tall weeds here. To break it down, "crush injury" for the purposes here is distinct from blunt trauma, although blunt trauma is obviously a major component in the crush syndrome. Crush syndrome or injury or whatever you want to call it is a constellation of problems that are superimposed on the blunt trauma problem, and, as your question suggests, does not require blunt trauma to set in motion. So....the problems we've identified so far...
1. Massive liberation of muscle protein, myoglobin, into the vascular space leading to renal damage/failure (rhabdomyolysis)
2. Sudden release of severe, blood flow restricting limb/pelvis compression after an extended period of entrapment. Accumulation of anaerobic metabolites and cellular release of potassium from cell death, (to say nothing of vascular injury and thrombus formation) is capable of causing sudden and catastrophic cardiovascular collapse via sudden and profound metabolic acidosis and hyperkalemia.
How can these present through a medical mechanism?
1. Rhabdomyolysis can occur when a poorly conditioned athlete attempts an activity that is far out of his depth, ie a marathon. Hyperthermic emergencies, diabetic emergencies, drug reactions, certain infectious diseases can all have the same effect.
2. As far as this goes, a scenario will be instructive...Say a poorly conditioned alcoholic is on a binge one weekend and spends the weekend on the couch drinking and goes into a fib (maybe has a history of p-afib). He ignores it and continues to drink until Monday when he sobers up. As the week progresses he notices pallor pain in both legs which he ignores for a couple of days until the pain is unbearable. He's admitted to the hospital for pulseless lower extremities 2/2 embolization of thrombus most likely caused by is immobility and a fib. Several days of no flow to both legs from a clot does the same thing as a two ton concrete block on the pelvis.
When the surgeon fishes out those clots and reperfuses the lower half of the body, that "acid wash" will occur and the consequences are the same as if he were extricated from a building collapse...get it?
Obviously, the severity of the syndrome with vary with the situation. While these things can occur, most times the degree to which they present are not clinically significant and resolve on their own with no treatment except rest and fluids.