What Happened: On Thursday, shortly after the cash market open, an options trader made a particularly large $8 million bet on the iShares iBoxx Bond ETF (ARCA: LQD). The trader placed a put spread on the December monthly op-ex (17th) at the $131 and $128 strikes for an estimated price of $0.80 on the $131 puts and $0.30 on the $128 puts.
This translates roughly to 160,000 options, of which you can see the large open interest at these strikes in the image below.