Whilst closeouts are triggered when the equity value falls to 50% or lower than the required margin on the account, the closing of positions is based on the prevailing market price. In fast moving market conditions, or where there is gapping in the market, this can result in positions being closed at a level significantly lower than the 50% level.
Example of a price change from $26 to $29.03:
Let's assume you held a short position of 100 shares on GME where the margin required to maintain the position over the weekend was $520. The equity on the account on the market close on Friday night was $420, so the account was on margin call for $100.
In the chart displayed you can see how demand for GME based on market sentiment, meant that the stock reopened significantly higher on Monday morning. Holding 100 shares of GME at 5:1 leverage would mean that the position ends up with a $303 additional loss leaving you with an equity of $117. This is 20% of the margin requirement ($580.60) and therefore has triggered a margin closeout.
However, the market could have opened $20 higher instead. In which case, the positions would be closed leaving a negative balance on the account. However as a retail client, you receive negative balance protection. This means your balance would be automatically reinstated to $0. You can find out more about negative balance here.
Additionally, if you require for your trades to be closed at specific levels, you may use risk management tools such as a stop-loss/take-profit or even guaranteed stops. More information can be found here.