Sell signals in leading stocks proliferated Friday as the market suffered back-to-back jarring declines. Even steady leaders like Starbucks (SBUX) and Nike (NKE), which were on the IBD 50 list Friday, broke badly. Starbucks crashed 5% in huge volume and sliced through its 50-day moving average, marking a sell signal. It was also the stock’s biggest one-day point drop in its run-up and signaled a change in character for the stock.