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 Williams Partners (NYSE: WPZ) somehow sidestepped yesterday's market massacre, levitating 2% in a turbulent environment. But the law of gravity will not be defied. Today, Williams is trading as much as 14% lower.

So what: Q2 earnings blew out the box Wednesday, contributing to yesterday's gravity-defying rise. Williams reported earning $0.91 per unit, versus $0.66 earned in the previous year's Q2 -- and versus Street expectations of just $0.38.

Now what: But if Williams did so well last quarter, why are investors now selling the stock? I'm taking a shot in the dark here, but could it be ... the price? Analysts on average think Williams will grow its earnings at about 7% per year over the next five years. Even after Q2's blowout results, though, the company still trades for more than 13 times this year's earnings -- and 14 times next year's haul, which analysts expect will be lighter than this year's. Result: The shares looked overpriced yesterday, and only fairly priced today.

But what if Williams continues to grow faster than analysts expect? Add the stock to your Fool Watchlist and find out what happens then.