Something odd is happening right when the stock market closes

The stock market has bounced back after the UK's shock decision to leave Europe, with many equity benchmarks right where they were before the vote.

Does that mean risk-taking is back? Nope.

Big-time traders are back in the market during the day, but they're also avoiding what is called overnight risk.

Credit Suisse took a look at the percentage of the day's US stock trading volume that is in the MOC print, or market on close. This is a kind of order that is executed as close to the end of the market day as possible.

The average volume has increased from 8.4% to 9.8% in the last two years, according to the Swiss bank.

That means that traders are frenetically reducing their positions right before the market closes.

Here's what it all means: