Ureteral Injury with Urinary Retention leading to Hyperammonemia: The pelvic crush injury could have caused ureteral damage, leading to urinary retention. The retained urine can become a source of ammonia, especially if there is an infection, which can lead to hyperammonemia. The sudden onset of hyperammonemia and tonsillar herniation suggests a catastrophic increase in intracranial pressure, possibly due to the systemic effects of hyperammonemia.

Hepatic Injury: A pelvic crush injury can also involve the liver, especially if the force of the injury was severe enough to cause intra-abdominal damage. Hepatic dysfunction can lead to hyperammonemia due to the liver's inability to detoxify ammonia.
Sepsis: The pelvic injury could be complicated by infection, leading to sepsis. Sepsis can cause multi-organ dysfunction, including hepatic dysfunction, which can result in hyperammonemia.
Rhabdomyolysis: The crush injury can cause significant muscle damage, leading to rhabdomyolysis. Although rhabdomyolysis itself does not directly cause hyperammonemia, the associated renal failure can lead to an accumulation of toxins, including ammonia.

Diabetic Ketoacidosis (DKA): Although less likely given the context, DKA can cause hyperammonemia and altered mental status. It's crucial to rule out DKA due to its high morbidity and mortality if left untreated.
Meningitis or Encephalitis: Infections of the central nervous system can cause altered mental status and increased intracranial pressure, mimicking the presentation of tonsillar herniation. These conditions require immediate antibiotic treatment.
Pituitary Apoplexy: This is a medical emergency that can cause sudden increase in intracranial pressure and altered mental status. Although rare, it's a diagnosis that cannot be missed due to its high mortality if not promptly treated.

Reye's Syndrome: A rare but serious condition that causes swelling in the liver and brain. It can be associated with the use of aspirin during a viral infection and can lead to hyperammonemia and increased intracranial pressure.
Urea Cycle Disorders: These are genetic disorders that affect the body's ability to remove ammonia from the blood. Although rare, they can present acutely in response to stressors like trauma and can cause hyperammonemia.
Hyperammonemia-Hyperinsulinism Syndrome: A rare condition characterized by hyperinsulinism and hyperammonemia. It's a consideration in patients with unexplained hyperammonemia, especially if there's a history of hypoglycemia.