Answer: a listing price that was too high for the market.
Step-by-step explanation:
An expired listing occurs when the house for sale was not sold in the time period that was allowed by the contract between the house owner and the sales agent.
If this was to happen then it could be because prospective buyers deemed the house to be worth less than it was being offered for and so did not buy it. Both the seller and the agent can learn from this as it very much could be an indicator that the listing price was too high for the market.