50 lbs of Ice falls from Sky in Iowa!

Did you see this? Amazing huh! Ice balls weighing 50 lbs pelted homes from the sky in Iowa!

There could be two reasons – either they fell from an airplane (one heck of chunks of ice that plane must be carrying on its outer body!!) or ice accumulated high up in the atmosphere. Both as you would have guessed would be VERY RARE indeed.. hahaha almost like impossible? So what was the reason? If it did accumulate that heavy why it didnt fall before it reached the weight of 50 lbs.