Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

The Dow Jones Industrials (DJINDICES:^DJI) fell 21 points today, giving back some of the gains that the average has posted recently after reports from Europe suggested the recovery there might not be as strong as investors had originally hoped. But even on this side of the Atlantic, Hertz Global Holdings (NYSE:HTZ), ImmunoGen (NASDAQ:IMGN), and Orbitz Worldwide (UNKNOWN:OWW.DL) all suffered drops of their own, each losing more than 10%. Let's find out why these stocks fell so hard today.

Hertz Global dropped 10.5% after it revealed in its third-quarter earnings report that Simply Wheelz, which had bought Hertz's Advantage brand, had filed for bankruptcy and therefore would be unlikely to pay Hertz properly for the acquisition. Yet even with Hertz's chances for recovery from the bankruptcy court looking grim and the company facing impairment losses of $40 million from the incident, the rental-car giant posted strong revenue gains and better earnings than expected.

ImmunoGen posted a 19% decline, as the company said this morning that its IMGN901 experimental drug to treat small-cell lung cancer wasn't producing improvements in the length of time patients went without the disease progressing. As a result, ImmunoGen followed the recommendation of its independent monitoring committee and stopped its phase 2 trial of the drug. The setback is significant for the company, but ImmunoGen has several other antibody-drug conjugate treatments in its pipeline to treat a variety of other types of cancer.

Orbitz Worldwide plunged almost 20% on mixed earnings results in its third quarter. Sales jumped but earnings fell from year-ago levels, as higher marketing costs weighed on profitability. A slight shortfall on sales guidance for the remainder of the year also disappointed shareholders. Given the challenges of competing against other giants in the online-portal space, Orbitz will need to find innovative ways to drive customer loyalty if it wants to avoid becoming an afterthought in the industry.