Large investors seem to trade baskets of stocks rather than individual stocks. Hence bad news can cause them to dump a whole sector rather than an individual stock.

Also these days people are very nervous about how earnings will be affected by the problems in Europe, etc, and possible recession there and maybe spreading here. So they take the actions or reports of companies as an indication of a trend and then use that as a basis to adjust their portfolio theoretically ahead of the market. Of course they are not always right. And hence the result is volatility in a nervous market.