Through the close of trading Thursday, the S&P 500 had been down eight consecutive days in a row. It's a rare feat, happening only twice in the last 20 years.
Whether or not the index closes down Friday (for the ninth day in a row), or finishes up to start a new positive streak, basic probability tells us that we would get these long streaks every once in a while. It's like a basketball team over the course of a season, a perfectly average team might still have a long losing or winning streak.
In the case of the stock market, we can show that its streaks aren't any different from flipping a coin. In fact, when we did simulate thousands of coin flips on a computer, we got the same number of long streaks as the real stock market has in the past 66 years.