Around 10% of the 20 million shares Microsoft head Bill Gates sold last month have been affected by the ongoing global stock market turmoil.

The company chairman sold nearly 4 million shares of Microsoft stock in the last three days of February, according to filings to the US Securities and Exchange Commission (SEC).

The sale coincided with one of the worst days that global stock markets have seen in years. The Dow Jones Industrial Average plunged 416 points on the day to end at 12,216. Microsoft dropped 4.1% on the day to end at $27.87 (£14.33).

Market watchers blame the global equities turmoil on everything from a sell-off in Shanghai shares to fears of an end to the carry-trade, a term referring to Japan’s interest rates. For years, investors have been able to borrow money in Japan for nearly free because the nation has maintained near-zero interest rates. Borrowers have then taken that money and invested it elsewhere, including in global stock and precious metals markets. The fear is that Japan may start raising interest rates, causing investors to sell assets bought using their borrowed Yen to repay their obligations. Japan has maintained low interest rates for years, a measure aimed at bolstering its economy.

No one has blamed the technology sector for the sell-off, but share sales by major figures do attract attention from people trying to gauge the direction that markets might take. Gates is considered an insider due to his position at Microsoft, and therefore has to file notices to the SEC when he sells shares, leaving a public record of transactions.

He collected over half a billion dollars from the February share sale, according to the SEC filings.

The filings do not say why Gates sold the shares. Even after the sale he remains the largest shareholder in Microsoft with 917.5 million shares, or around 9.2% of the shares outstanding.