Let's say there is a stock of ABC currently at $8, and I sell a (naked) call option on it, with a strike price of $10 and expiration in two months. Suppose my broker lets me do this if I have 50% of the stock value in my margin account - in this case $4.
Now what happens if for some reason the stock price begins shooting up rapidly (in a span of few hours) from $8 to, say, $30? What can my broker do to prevent liability?
It can't force me to buy the stock to cover the position, because I only have $4 in the margin account and the stock price is now $30.
It can't buy back the call option contract, because it is now worth at least $30-$10 = $20, which is still more than the $4 I have.
But if it doesn't do any of the above, then at the end of the day, who will pay the holder of the call option when he attempts to exercise it?