1) Zillow made a financial/ML model which would predict the sell price of a home in N months.
2) Zillow leveraged dirt cheap mortgage rates + VC money to outbid other sellers to be able to sell the home in N months
3) Zillow drove property developers and house flippers out of the market.
4) The homes Zillow is looking to sell within N months don't have the "improvements" to justify the increased price.
5) The Zillow financial model turns out to have been biased on historic trends.
As others have noted Zillow makes money on more than just the price delta, they are also deploying capital to acquire more of the market to collect fees from. They are also theoretically able to benefit from appreciating prices being much much higher than interest costs in 2021.
The con of all of this is that it's a rent seeking business model entering markets that are already full to the brim with rent seekers. If Zillow acquires enough of the "float" in the market they can effectively set prices to be whatever the demand will bear. Surprisingly housing is so constrained at this point that in a major city like Boston you could purchase all outstanding homes for <200 Million dollars.