Just because your car has been repossessed doesn't mean you don't still owe the bank money on it. If that vehicle gets sold at auction for less than the bank is owed on it, the difference is called the "deficiency." And as MSNBC informs, banks are increasingly suing to get that deficiency amount back, plus applicable attorney's fees.
With the increasing cost of vehicles, smaller down payments, and longer loan terms, it is easier than ever to get upside-down on your car loan. Furthermore, banks don't send a repossessed car to auction and work to get the highest price for it. They farm it out to third parties who need to move cars, not hold out for the absolute best deal. Take depressed auction prices into account as well, and it's not hard to end up with a deficiency in the tens of thousands of dollars.
Banks who win deficiency judgments can sometimes garnish wages and force the sale of property, but again, that's only in certain cases. The best ways to ward off a deficiency lawsuit are to get the shortest car loan term you can afford, which will help avoid getting upside-down in the first place. If you still can't manage, try and sell the car yourself, since you'll likely get a much better price than the bank will at auction (and you'll avoid a repo on your credit history). Failing that, bankruptcy should clear the debt as an unsecured loan.