The two below cases may apply to your situation. Please check to make sure you did not set up any transactions.
1. After placing an order, an applicable buy/sell order matching the value you set did not appear and the order was left on the marketplace without being erased.
2. You set a limit value when purchasing another currency or item and that limit value was met. *If a limit is set, a buy/sell order will be conducted automatically after a trade is established.