Faced with Professor McGonagall's inquiry, Albert quickly repeated what he had just told Madam Pomfrey. As for why the giant snowman had suddenly collapsed, it was naturally because Professor Snape had cast a counter-curse on it.
Being able to build such a massive snowman was naturally because Albert had used magic on it during the manufacturing process. Once those spells were undone, the giant snowman naturally collapsed.
Moreover, Professor Snape hadn't listened to his warning and failed to dodge in time, ending up crushed by the snowman—he really couldn't be blamed.
Chapter 348