For what it's worth...
Good friends with a PICU MD here in AZ. He is reporting the same thing and science hasn't been able to determine why yet, it's still early.
Also, have a 22y/o son that lives here in town and appears to have it. Strapping, healthy and athletic... or at least was. He's been hard down for about 5 days now. PICU doc has arranged testing tomorrow.