What a margin call is
A margin call is a warning that your losing positions have eaten into the funds backing them. Your 'margin level' (equity divided by used margin, as a percentage) has fallen to the broker's call threshold. It is a signal to add funds or reduce risk before things get worse.
It is not yet a forced closure — that is the stop-out, which comes at a lower level if you do nothing.
Why it happened
Positions moved against you and unrealised losses shrank your equity. You were over-leveraged — too large a position for the account. You had several correlated trades all losing at once. Or swap and other costs quietly reduced your equity over time.
How to respond
Do not panic-add money to defend a bad trade. First, look at whether the positions still make sense; often the right move is to reduce or close the weakest ones to restore your margin level. Cutting size is usually safer than adding funds to a losing position.
If you do add funds, add only what you can afford to lose, and understand you are increasing your exposure, not fixing the underlying trade.
How to prevent it
Use modest leverage, size positions so a normal adverse move cannot threaten your account, keep free margin in reserve, and use stop-losses. Margin calls are almost always a symptom of positions that were too big.
Broker's fault or yours?
A margin call is the mechanics working as intended, driven by your own position sizing — it is on your side. It is best treated as a hard lesson about leverage rather than something to blame the broker for.