Good question. The drops affected a number of the MLPs including EPD, EPB, BPL, KMP and other stocks in the oil/gas patch. There was no particular event that I can find. I don't think the turmoil at CHK contributes to this. Perhaps there is a belated realization that oil is weakening (and that natural gas is weak), and the potential exists for a slump in demand because of the EU chaos. Plus the stocks had weakened a bit already and a bunch of computers saw prices hitting their sell limits. But it's still pretty interesting that these very big drops happened with virtually no news or commentary. I would think the pipeline stocks shouldn't be heavily hit by the ups and downs of oil/gas prices though. So it's still a mystery.
It is perhaps a sign that investors are preparing for a global credit crunch/financial crisis if and when Greece defaults/leaves the Euro. In 2008/2009 MLPs were hammered, in part because they depend so heavily on credit. MWE, e.g., was in the single digits.