When an account is in a margin deficit, any open position can be liquidated by IBKR to return the account to margin compliance, including currency positions. In addition, IBKR may act to convert non-base currency balances as described in the How can I close out the nominal FOREX positions in my account? knowledge base article.
Why was my currency position liquidated?
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Why am I unable to withdraw funds?
How do you choose what to liquidate and why?
Why did I receive an execution when I did not enter an order?
How do I close a currency balance in IBKR Mobile?