This was most likely caused by a 'Margin Call'. A margin call is when the available funds you have to trade (Available Margin) has dropped to a level which has triggered the closure of your open trades.
You can see how much margin you have on the positions page...
The level this trigger is reached is typically when 'used margin' reaches 50% of total margin.
By way of warning, you will normally receive emails that margin calls are approaching. To avoid having your trades closed, you will need to add funds to your account.