are subject to "market forces." Meaning that if players are shorting or selling off a stock, then the price may drop at some point. The opposite is also true...if players are buying or covering stocks, they price will rise.
Prices on stocks will fluctuate based on what players are doing. Perhaps people sell off a stock when it hits 'Wrap' as they may have made as much $$ on it as they want, and they sell off to use their profits on other things. Maybe there is news about the stock that players made trades based on that news. Perhaps the movie will open "limited" rather than "wide" so players may sell off because of that..
There is no hard and fast rule as to why stocks go up or down in price - again, it's based on the market and how purchases or sales are being recorded.