Here's what this could mean for investors in the world's second-largest communications services stock by market cap. Why SpaceX's lockup expiration could move its stockStock prices, like the prices of any product or service, are governed by the law of supply and demand. It's not unusual to see early employees wait until they can sell shares, then leave for another job with a rival. Is a steep sell-off of SpaceX stock a foregone conclusion after Thursday? Insiders could wait to sell shares on one of the other lockup expiration dates.