Why did the market order in the pre-open session change to a limit order?

The pre-open session, captures all orders to determine the equilibrium price during the session. This happens to ensure fair price discovery and to prevent market orders from executing at extreme or unfavorable prices when regular trading begins.

If any market order is not fully matched during the pre-open session, the remaining quantity is placed as a limit order at the equilibrium price, allowing for better control and stability in the market. This mechanism helps maintain orderly trading and protects traders from price volatility at the market open.          

In case, when opening price cannot be determined, the market orders will be carried over at the previous day closing price.