Why is China's stock market rapidly crashing?

China’s CSI 300 index, which tracks 300 major companies listed in Shanghai and Shenzhen, entered a bear market on Thursday after falling 20% from its June peak, then dropped another 3% on Friday and recorded a tenth consecutive weekly decline. The slide reflects investor frustration with a struggling economy, expensive artificial-intelligence companies, and Beijing’s efforts to keep activity going. Retail investors account for close to 90% of trading in China, HSBC estimates, compared with 20%-25% on the New York Stock Exchange. Beijing has cut public spending for seven straight months as of September while introducing targeted measures including mortgage subsidies and 550 billion yuan in additional local-government bonds.