What happened

Shares of Tesla (NASDAQ: TSLA) tumbled again on Friday. As of 12:27 p.m. EDT, the stock is down more than 1%, trading at about $400, but it declined to as low as $372.05 at one point during the trading day.

The stock's pullback extends a downtrend for Tesla so far in September as many growth stocks have been falling sharply following a huge run-up in their share prices since April.

Tesla's Model S, X, 3, and Y

Image source: Tesla.

So what

Tesla stock is down more than 15% in September.

It's not surprising to see Tesla investors taking their profits, as the stock rose 500% between the beginning of the year and the end of August. Tesla's rapid gains in August, in particular, were arguably becoming irrational, as the primary news seeming to drive the stock higher was a stock split -- a structural change to shares that, in theory, does nothing to make them more valuable.

Tesla's stock has also likely been declining because of a broader market sell-off in recent trading days that has been largely driven by profit-taking in growth stocks. Many growth stocks have significantly outperformed the market this year and it's reasonable to see them taking a breather after their impressive gains.

Now what

Investors should expect more significant volatility in Tesla shares in the coming days, weeks, and even months. Given its pricey valuation, which prices in years of rapid business growth, changes in investor sentiment about the company's long-term potential can have huge impacts on the company's share price.