The cause for the elevated potassium is pseudo-hyperkalemia.

The key clue to the diagnosis is marked hyperkalemia with a normal EKG and telemetry tracings.

The physiology behind this in this situation is potassium leaking out of platelets and WBCs within the blood tube.  (Note how elevated the platelets and WBC counts are.)  Other potential causes of pseudohyperkalemia are in vitro hemolysis (overall much more common – but often easier to recognize as most labs will report out the sample as hemolyzed) and prolonged tourniquet application.

A blood test run on a bedside iSTAT (processed immediately) shows a normal potassium.

This is an important diagnosis, because treating this as hyperkalemia (e.g. with insulin, dextrose, diuretics) would risk actually inducing hypokalemia.  

For more information on pseudohyperkalemia and the evaluation of hyperkalemia, see the IBCC chapter here.