The electric vehicle stock is falling further because of one big reason.What goes up must come down. Rivian Automotive (RIVN 0.48%) stock's jaw-dropping rally in the three months through July appeared unsustainable, and that's what investors are witnessing now. The electric vehicle (EV) stock has slumped by double-digit percentages for two straight months now. After losing nearly 14% of its value in August, shares of Rivian have crashed another 20.6% in September, according to data provided by S&P Global Ma ...