Quote from Ghost of Cutten:
The short answer is that any time you have a short-leg in an American-style option, you have open-ended risk due to the possibility of early exercise and overnight gap risk. You could theoretically lose more than your account value, and your broker would be on the hook.
The lesson is to know your broker's margin call and liquidation policy before, not after you start trading. And to realise that any time you short an American-style option, even if it's part of a spread, you have open-ended risk.
Was this IB, by any chance? Anyway, try arbitration if the amount was big enough.