Three Cheers: Heavy vs Shield?

I recently played a game of Three Cheers for Master in which I caused a heavy minion to fall on top of another minion who had a shield icon on top.

I said that the shield only blocks attacks, and a fall is the result of gravity... Not an attack. So it counts, and the minion underneath should be crushed.

Her response was that there is no clear distinguishing in the book that the "fall" is or isn't an attack, and therefore the shield should defend against a fall.

Who was right?!

You are right. Shields only block aggression, not gravity.