For instance, suppose they build 100 apartments in the suburb of X. They know that for every $1k the price of housing in X drops, they lose $100k. So weirdly, they tend to leave housing vacant and sell it at a drip-feed so that they're always selling less houses than there are buyers - there is a huge difference between selling 3 apartments to 4 buyers, vs selling 4 apartments to 4 buyers. This particularly bad when there are few, large developers instead of lots of small developers, as the few large developers can collude to price-fix much easier.

Please don't interpret this as contradicting the fact that California needs to massively overhaul its zoning/construction permit system - the fact that it can take years to receive permission instead of weeks is a big reason for the price of housing (and raises capex requirements which results in fewer but larger developers, see previous paragraph). Ditto for all the zoning that mandates exclusively single-family homes, and other such pointless exclusionism.