At the end of a BMWFS lease the car is returned to a dealer if the leasee does not buy it off lease end. If the dealer does not want it, then it USUALLY ends up at an auction.
With a 3rd party lease often the cars are turned in at the auction location itself, not to a dealer.
There could be some cars that show up that were traded in at a non-BMW dealer. Say the guy wants an E55 AMG or a loaded LS430. The MB or Lexus dealer does not really want a BMW on the lot. They may offer it to a BMW dealer if one is nearby, or (if they have low value in the trade) just roll it through the auction.
There are too many variables to state that auction cars are bad. A good auction should have the CarFax available to see if it was a lease car. It is always buyer beware - and the trouble with an auction car is no time/way to get a shop to inspect it before you buy. But if you get a car for pennies on a dollar there may be room to fix a few things that could be wrong.