From what I've read there are analysts who estimate what the performance of the past quarter is for most companies. Sometimes there are several analysts. Investors buy or sell stock based upon these estimates.
If news comes out that is different than these estimates, the value of the stock changes once this is found out. That is why if a company's estimated loss during a quarter was quite bad the stock price can actually rise after the results are released if those results are better than estimates. The price was factored in previously from estimates and now the real performance is better.
I've read that just more than 50% of companies' results released this year have been worse than estimated. It will be an interesting week, but yes it looks like a scary week