Here's a quick summary of how the process for determining the opening price works. The basic problem is that the closing price from the previous trading day is no longer a valid indicator of a stock's perceived value. News may have appeared since the previous close, there may have been trading on foreign exchanges that open before US domestic exchanges, and there surely has been a flow of new and changed orders since the previous close. But it is possible that today's book contains no orders from yesterday - or at least none that might affect the opening. So the specialist may have to make an educated guess to kick off initial trading.
Market makers perform a two stage round-robin opening.
First, each posts a single bid and asked price pair. This price can signal each firms view of the security, its current desire to buy or sell, or it may indicate that a firm is out of calibration with others in the market. And off the day's trading goes. This commonly happens when news that was released after the previous market close impacts a security's price. The opening price in these cases differs sharply from the previous day's close, either higher or lower.