What about going only -200% let's say on one account and a person has only one of them - their next trades would still be plummeted by this loan that wasn't satisfied?
if we're talking about cross margin account for future trade usually the exchange liquidate when there's not enough money in the account balance to make sure that there's money to pay so once it hits around -90% to -100%, if there's no money left, the exchange will automatically close your position and fulfill the contract.
that's just a way of exchange or any broker to give you leverage without the complexity but also impose stricter rule so they aren't on the losing side having so many people default on loan I guess. you're only allowed to have more than -100% PnL if you have money to compensate for your current position.