A number of years ago there was a thread about exercising of some call options held in an IRA account. They were exercised and there were insufficient funds to cover the called stock. So IB sold on the open on Monday. The stock sold for less than the exercise price. The account took a hit greater than its equity. Thus, a debit balance. The account owner had to pay up. The account owner was also probably fined by the IRS for an excess contribution as the debit was greater than the allowable IRA contribution at that time.
Jack