Shares of Whiting Petroleum Corp. (NYSE: WLL) fell more than 15% in early trading Friday to $4.15, after closing at $4.91 on Thursday. Volume was very heavy, with about 12 million shares traded in the first hour, compared with a daily average of about 16 million.

The likeliest suspect is a downgrade from Moody’s Investor Services on Thursday, which took Whiting’s corporate family rating from Ba2 to Caa1, a drop of five notches deeper into junk territory. Here’s Moody’s rationale:

This reflects Moody’s expectations of very weak cash flow-based leverage metrics in 2016 and particularly in 2017, when its hedges roll off. With the company facing structurally low oil prices through 2017 and a heavy debt burden, there is a heightened risk of a debt restructuring. Whiting’s Caa1 CFR is supported by the company’s reserves and production scale and its long-lived reserves profile. While we recognize the steps that Whiting has undertaken in response to weak pricing levels, we believe the ability to further adjust to a lower price environment will be more challenging and the timing and execution of assets sales more uncertain.

Another possible suspect is a Schedule 13G/A filed on Thursday revealing that Vanguard Group acquired a 6.97% stake (14.25 million shares) in Whiting as of the end of December 2015. The Vanguard Energy ETF (NYSEMKT: VDE) held 701,425 Whiting shares at the end of December.

So, is Vanguard going to add more than 14 million shares to its energy exchange traded fund? Probably not, because its largest holding, Exxon Mobil, tallies just 13.8 million shares. Besides, Whiting stock is down 88% over the past 12 months, and Moody’s doesn’t see any near-term improvement.

One possibility is that Vanguard is looking at Whiting as a component of a Core Bond Fund the firm is expected to launch sometime this quarter. The new bond fund will try to “outperform the broad investment-grade U.S. fixed-income market” by investing in “a broadly diversified portfolio of high-quality bonds, including Treasury, mortgage-backed, and corporate securities of varying yields and maturities.” Vanguard filed its registration statement with the U.S. Securities and Exchange Commission (SEC) on December 21.

While the Moody’s downgrade is probably the main reason Whiting stock traded down about 8.5% at $4.49, Vanguard’s filing may underline it. Whiting’s 52-week range is $4.01 to $41.57, and the consensus price target on the stock is $15.25.

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