A broker requires a minimum equity level against borrowed funds. When losses take the account below it, the broker calls for more cash or securities, and the deadline is short, often the same or the next trading day.
Not meeting it hands the decision to the broker, who closes positions at market to restore the ratio. The choice of what is sold, and when, is no longer yours.
Margin calls cluster, and that is what makes them a market event rather than an individual one. A sharp decline triggers them across many accounts simultaneously, the forced selling deepens the decline, and the second wave of calls arrives on the back of the first.