It's because of the F.E.A.R. set. It's basically where you have a Level 1 Aron, with Sturdy, Endeavor, you have Sandstorm up, and you are holding the Shell Bell or Leftovers. Since you are Level 1, you'll get OHKO'd by anything, except since you have Sturdy, you won't die. You use Endeavor, and your opponent gets down to 1 HP. Then the Sand finishes them off.
NOTE: I commented on this on the tier page here.