What: Units of Plains All American Pipeline (PAA 1.14%) and its general partner Plains GP Holding (PAGP 1.25%) both slumped more than 11% by 1:15 p.m. ET on Monday. Fueling today's sell-off was another drop in the price of oil, as well as an analyst downgrade.

So what: The price of crude slumped nearly 3% by the early afternoon pushing it down to around $30 a barrel. That drop weighed on the entire energy sector, with dozens of energy stocks, including Plains All American Pipeline and Plains GP Holding, heading lower.

In addition to that, Plains All American Pipeline was downgraded by Robert W. Baird from outperform all the way to underperform. Furthermore, Baird slashed its price target from $25 all the way down to $16. That's a pretty steep downgrade and reflects the view that weakening oil prices will have a big impact on Plains because low oil prices could lead to lower oil volumes being transported by the company.

That said, it is worth noting that both Plains All America Pipeline and Plains GP Holding will report fourth-quarter results after the market closes today. The uncertainty of what's in that report is another thing that has investors worried today. However, the report will be what actually sheds light on how the worsening downturn has affected its business.  

Now what: While there isn't a whole lot of visibility in the oil market right now, investors will get a much clearer picture of Plains' recent performance once it reports earnings. That report, along with management's outlook, could calm investors' fears -- or make them worse. Because of that, investors might want to wait to see what's in that report before reading too deeply into today's downgrade-driven sell-off.