It doesn't matter what we call it.

The current price is based on a prevailing belief among enough market participants who buy homes because they think someone will pay them more for it later, and/or that it will have positive cash flow regardless of the cost.

To answer you specifically it's a bit of a combo of the total drop in real prices and the rate at which it happens.

When current prices are not based on fundamental value but rather the belief that a bigger fool will pay more, and the last fool is exhausted, then the illusion supporting the price goes away in the blink of an eye -- as fast as a thought can change in a human mind.

Hence it "pops!"