The lowest possible price is also the point where the value of your portfolio is lowest and the margin call is most severe. It is completely normal for brokers to immediately close positions when you are highly leveraged and the market is moving quickly on a direction which might leave your portfolio at a negative value - they don't want to be left holding the bag if you take too long to respond. Generally your broker will only give you time to correct things when volatility is low enough that things won't change over the next few hours.