For example, if an exchange's max leverage was 30x and due to an adverse price move a trade became 30.1x leverage, then it could be liquidated by anyone. Trades can be liquidated when they do not have the minimum amount of collateral (for example 100 USDC) left in them because the liquidator needs to be able to earn enough of a fee to pay for the gas of liquidating the trade. If there was no minimum collateral amount, someone could attack the protocol by opening up a significant number of tiny trades that would not be economical to liquidate.