On CNBC's "Options Action," Mike Khouw said VanEck Vectors Gold Miners Etf GDX traded two times its average daily put options volume on Monday. A lot of that activity was concentrated in the September 24, $28-strike puts as over 3,000 contracts were traded for 20 cents. Buyers of these puts are betting GDX could decline below $27.80, which would represent a decline of more than 14%.