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

What: Shares of bond insurer Assured Guaranty (NYSE: AGO) were declining for a second day in a row, shedding as much as 11% on heavy volume.

So what: Of the major bond insurers, Assured Guaranty was the only one to make it through the financial crisis in relatively strong condition. So what's its excuse for the retreat? Standard & Poor's is making changes to the way that it rates bond insurers and it could result in a cut to Assured Guaranty's rating if it doesn't either raise capital or reduce its risk. For obvious reasons, this has rubbed investors the wrong way. Fellow bond insurer MBIA (NYSE: MBI) was also down nearly 10% today since it'll face the same decision if it wants to hang on to its current ratings.

Now what: Looking back at the wreckage of the financial crisis, along with the current condition of MBIA and the recent bankruptcy of Ambac, it seems like S&P was due to take a look at how it was rating bond insurers. The changes could sting for Assured Guaranty in the short term, but the company proved its mettle during the crisis and now holds a strong position in the industry, so investors may not want to be so quick to head for the hills.

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy prefers dividends over a sharp stick in the eye.