Right. That "spike down" is a drawdown (not "intraday drawdown") and can cause a margin call.

There are subtle differences between drawdown, intraday drawdown, balance, and equity. You have to know them for knowing your risk. They are explained here: http://zorro-trader.com/manual/en/performance.htm. Look into the example under "Remarks".

As a rule of thumb, a real drawdown is bad for your account, an intraday drawdown is bad for your nerves.