Does some trading take place overnight, maybe internationally? Are there any other factors?
The bid/ask can change overnight, as investors change their mind about what a stock is actually worth while the exchanges are closed. The reported price is just the last 'ask' that went through an actual trade. If a value of a stock drops after hours, then the next day, demand for the stock (and therefore the 'ask' price) drops. Thus, the price change.
American stocks do trade overseas (sort of), but the bid/ask there may or may not reflect the bid/ask of domestic exchanges. Stocks tend to reflect foreign trading prices once the market reopens the next day, but there is no requirement that the first trade price of any trading day reflects the prior day's close. The only thing the price reflects is what investors think a stock is worth at that very moment.
The biggest factor is changes in investor demand while the exchanges are closed.
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