I guess you can not sue them because they can liquidate the shares for many reasons. One of them is that you get a margin call. The other one is that the shares get called. If even one share was traded high enough to make your account margin requirement negative, they can liquidate some shares. In your case that they liquidated lots of shares. In my IB account, if the liquidate me , they do it at 15:50 if my SMA is negative or right away if my account goes negative but they do it to the amount that make me safe. I have accounts with many brokers. TD, Ameritrade, Scottrade, Fidelity. They all send you a polite letter about your margin situation and that was more dangerous because the problem might escalate. I believe if it is your habit to squeeze your account , change your broker with one of the ones that I mentioned.