Markets can stay irrational for longer than you can stay liquid.
So lets say you wanted to risk it and opened a trade and your margin level was 105%. You hoped that as you made profit your margin level will slowly grow but then the market became irrational and started moving against you.
Your margin level started dropping to 90% then 70% then 50% and your broker sent you a margin call warning you to deposit some more money or close some losing trades.
You refused and hoped the tide will change and your trade will become profitable, but the market stayed irrational and kept moving against you till your margin level fell to 20% which happened to be your brokers stop out level.
Your trade was closed forcefully by the broker and your account balance nearly wiped out. This story is a a reminder that the market can stay irrational for longer than you can stay liquid.