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.

For the S&P 500 (SNPINDEX:^GSPC), Tuesday brought yet another all-time record high, with the index gaining almost 10 points to finish near 1,772 as investors used their glass-half-full approach in expecting the Federal Reserve to delay pulling back on its quantitative easing. Yet even the positive mood didn't stop American Capital Agency (NASDAQ:AGNC), Blyth (UNKNOWN:BTH.DL), and Plum Creek Timber (UNKNOWN:PCL.DL) from posting significant declines. Let's take a closer look to find out what sent those stocks sharply lower today.

American Capital Agency dropped more than 8% as investors reacted negatively to the mortgage real estate investment trust's third-quarter results. A big decline in net interest spreads reflected the mREIT's decision to shorten the maturity of its mortgage-backed securities portfolio, and that raised fears that American Capital Agency will have to reduce its dividend again even after two payout cuts earlier this year. The silver lining is that the mREIT's book value fell just 1% after a much larger drop in the second quarter, and American Capital Agency is now better protected against further possible gains in interest rates. Still, dividend-hungry investors weren't willing to look beyond the immediate potential impact on earnings.

Blyth dropped almost 7% on an unusual day, with shares giving back a sizable portion of their 21% gains on Monday after the direct seller of candles, fragrances, and weight-loss products received an unsolicited buyout offer. The $269 million bid from CVSL, a peer in the health, home, and beauty direct-sales industry, amounts to $16.75 per share. Although some profit taking is normal after an acquisition bid, the shares now trade at an almost 14% discount to the buyout price. The wide discount suggests that shareholders don't believe Blyth will accept the offer and that no higher rival bid will materialize.

Plum Creek Timber also fell almost 7% as investors reacted to weak guidance for the fourth quarter as well as the timber REIT's intention to make a secondary offering of shares. The offering will help Plum Creek pay for its $1.1 billion purchase earlier this week of almost 1,000 square miles of industrial timberlands and rural land assets from MeadWestVaco (NYSE:WRK). Despite recent volatility in timber prices, Plum Creek anticipates that adding the land to its extensive portfolio will only help its cash flow situation improve in the years to come, especially with associated wind and mineral rights sweetening the deal.