House prices drop when desperate sellers arrive on the market, and when there are few buyers for them.
The factors contributing to this are many, but ultimately the seller is in a cashflow squeeze for whatever reason, and needs the cash now.
Agents like to call them a "motivated seller".
A very good observation.
In free (or mostly free) markets, the price is set at the margins of the market.
At any given point in time, most people are not buyers or sellers (save for traders). They are either holders (people who own assets and are not selling them), or they are lookers (people who are may one day want to buy more assets but aren't actively putting offers in [whether they already own some or not]).
If you're a holder or a looker you have ZERO impact on the market price.
It is the interplay of only those on the margins of the market (people actively trying to sell / people actively trying to buy) that sets the market price. That is demand and supply in action.
What happens next is a little bit chicken and egg, in that it could be desperate sellers who lower prices, or cautious buyers who offer lower prices. In truth it is a bit of both.
And it is the opposite during times of growth.
And while this is all patently obvious, it does highlight one thing.
Prices aren't set by the majority. They're set by that small percentage of economic agents who are active in the markets at any given point in time.
I've heard reports that 1 in every 9 properties in Queenstown (NZ) is listed for sale atm. That's a huge number, but it still is a small minority of property owners.