How can a reperfusion injury be lethal?

Organ failure. Reperfusion injuries are rare, complex. Loss of blood flow comes with swelling and increased pressure in limb. The leg's fascial compartments can not expand. When blood flow is restored and compartments released, reperfusion occurs. The metabolic waste products previously trapped, now get out of the leg. These waste products can overwhelm the other organs, shutting them down.
Hyperkalemia. Reperfusion of a severely ischemic limb like the leg washes all the toxic breakdown products of the ischemic injury into the circulation. Potassium is one of these and so the patient can become very hyperkalemic (high blood lveels of potassium) and this can cause fatal cardiac arrhythmias.