September has historically been a very volatile month for the stock market, and corporate executives booked some profits in anticipation of it.
According to data compiled by TrimTabs, insider selling reached $450 million daily in August through last Thursday, the highest level this year.
"As corporate buying is at least taking a breather, corporate insiders are ramping up share selling as the major U.S. stock market averages are at or near record highs," TrimTabs wrote in a note.
The and Nasdaq Composite both reached all-time highs to end August, boosted by robust economic data and sharp gains in tech.
Last week, the Commerce Department said the U.S. economy grew by 4.2 percent in the second quarter, up from an initial estimate of 4.1 percent. That marks the fastest growth rate for the U.S. economy since the third quarter of 2014.
"The economic fundamentals are overwhelming the potential negative impact of rising interest rates and the threat of a global economic slowdown due to the unresolved conflicts over tariffs and trade," Bruce Bittles, chief investment strategist at Baird, wrote in a note earlier this week. "The strength in the U.S. economy accompanied by rapid profit growth supports the long-term prospects for a continuation of the secular bull market."
Meanwhile, the S&P 500 tech sector rose 6.7 percent in August, its biggest monthly gain since May, when it rose 7.1 percent. Tech got a boost from Apple, which jumped nearly 20 percent last month and became the first U.S. publicly traded company to reach a $1 trillion market cap.
But history shows September is usually the worst month for stocks, possibly explaining why corporate executives sold so much stock last month. Data from the "Stock Trader's Almanac" show the S&P 500 and Nasdaq both fall an average of 0.5 percent in September. The Dow Jones Industrial Average, meanwhile, averages a loss of 0.7 percent in September.