The Nasdaq fell 5% on Thursday, its worst decline since March, and the S&P 500 had its worst session since June, but no one was quite sure why.

What happened: Fund managers and strategists posited that profit taking or rebalancing was to blame as no fundamental drivers for the sell-off were apparent and it remains unclear whether Thursday was a fluke or the beginning of retrenchment from what most Wall Street analysts viewed as an overextended market.

What we're hearing: "Today's swing was steeper than the markets have become used to seeing but we should expect more choppiness going forward, not less, given pandemic, economic and election uncertainty," Nela Richardson, investment strategist at Edward Jones, tells Axios.

Getting technical: Bloomberg's Vildana Hajric and Katherine Greifeld note that the S&P was trading around 15% above its 200-day moving average. The last time it reached such heights was January 2018, when a massive volatility shock sent the index into a correction.

• Sentiment also has gotten lopsided, with many investors sitting on the sidelines while those that have ventured into the market are almost uniformly bullish.
• Bearish bets on NYSE-listed stocks had fallen to a six-year low in mid-August and Citi's panic/euphoria model is showing one of the longest runs of extreme bullishness since the early 2000s.

The intrigue: No other assets responded strongly to the stock market's swan dive.

• Gold, a historic safety play in times of market distress, fell for the second day in a row, and silver prices declined by nearly 2%.
• WTI crude oil dipped just 0.25%.
• The dollar, which has sunk as the market has risen since the start of the third quarter, was little moved, edging lower by 0.1% on the day.
• The Japanese yen and Swiss franc, both traditional currency safe havens, gained just 0.1%.
• U.S. Treasury bonds were little moved, with yields on the benchmark 10-year note declining by just 3 basis points, about the same as the previous day when stocks rallied.

Where it stands: "The underlying feeling is that this was long overdue in the stock market," Kathy Jones, chief fixed income strategist at Schwab, tells Axios.

• "If this were to continue I expect you’d see more of a bond market reaction, but a 3%, 4%, 5% decline after a 60% run-up [in stocks] since March is still a pretty modest pullback."

What's next: Analysts pointed to apprehension about today's U.S. nonfarm payrolls report as a possible reason for the pullback.

• The average estimate for the report is 1.35 million jobs added, but economists' estimates in the Bloomberg survey range from an increase of 2.1 million to a decline of 100,000, according to DRW Trading.