Oil prices retreated again this week, falling by more than 4%, and hitting a fresh three-month low. This weakness put a lot of pressure on oil stocks, sending shares of several oil companies down by double digits. According to S&P Capital IQ data, Atlas Resource Partners (NYSE: ARP), Penn Virginia (NYSE: PVA), and BreitBurn Energy Partners (BBEPQ) were the biggest energy movers this week.

While weak oil prices were clearly what sent Atlas Resource Partners, BreitBurn Energy Partners, and Penn Virginia lower, it wasn't the only factor in the double-digit drubbing of these stocks. In Penn Virginia's case, the sell-off was a bit more company-specific, as it had news that investors didn't like.

However, nearly every other double-digit loser this week had one particular trait in common: Companies organized as an upstream MLP were among the hardest hit. It's what really fueled the double-digit sell-off in not only Atlas Resource Partners and BreitBurn Energy Partners, but four other upstream MLPs. To learn more about why these stocks fell so sharply this week, check out the slideshow below.