Here's Why PayPal, Visa, Discover, American Express, and Mastercard Are Plunging on Monday

To say that Monday is a volatile day in the stock market would be a massive understatement. After plunging by about 10% at the open and triggering a market circuit breaker, the major market indices have rebounded a bit. As of 11:15 a.m. EDT, the Dow Jones Industrial Average and S&P 500 benchmark indices are down by roughly 7% and 6%, respectively.

Unlike during last week's sell-off, the financial sector isn't doing too much worse than the overall stock market, with the Financial Select Sector SPDR ETF (NYSEMKT: XLF) down less than 8%.

Payment processors are a mixed bag. Visa (NYSE: V), Mastercard (NYSE: MA), and PayPal (NASDAQ: PYPL) are all down by about 7%. American Express (NYSE: AXP) is getting hit a bit worse, down 9% for the day. And Discover Financial Services (NYSE: DFS) is by far the worst-performing stock in the group, with shares down by 18% so far this morning.