Reduced interest rates. The price of riskier investments has a tendency to rise with reduce rates. Currently, the federal cash rate is larger than it has been for more than ten years and growing since the Federal Reserve tries to conquer back inflation. Short-sellers generally borrow shares from brokers. https://financefeeds.com/voo-stock-price-a-glimpse-of-vanguard-sp-500-index-fund-company/