I suspect I don't understand this very well,
If the market moves away from the shorter and there's insufficient maintenance margin, then the blockchain will try and satisfy the collateral need by withdrawing from the account automatically. Formerly, I believe there was also a 5% fee attached to this event to encourage traders to always keep sufficient balances.
However, if there's no BTS (or other backing) in the account, then this action will fail, in which case what happens to the created BitAsset?