Why did APP move on Sep 5, 2025?

AppLovin (APP) stock surged approximately 7% in after-hours trading on September 5, 2025, following the announcement that it would be added to the S&P 500 index. S&P Dow Jones Indices announced after the market close that AppLovin, alongside Robinhood Markets (HOOD) and Emcor Group (EME), would join the index prior to the open of trading on Monday, September 22, 2025. This highly anticipated index inclusion served as a major catalyst, driving expectations of massive passive buying pressure from S&P 500-tracking ETFs and mutual funds.