I think this is because of the nurses (and all Brits) feelings about royalty. Brits look at the royals differently than we do. Her embarrassment must have been more than she could stand. Terrible that she could not deal with it any other way.
In digging a little closer to this story, I have found reports that Prince William went absolutely nuts over this silly prank. He was demanding an investigation of the hospital. So perhaps in the end, it was the Royal Family - once again - behaving badly.