If you see that a transaction has taken place without your knowledge or consent, this is usually due to the processing of a (mandatory) corporate action. Examples of such events are ISIN or name changes or (reverse) stock splits.
Whenever a corporate action takes place, we process it through a simultaneous purchase and sale for the relevant financial instrument. You can recognize such a transaction by the fact that it takes place at the same time (e.g., at 00:00:00) and that the transaction is for the same amount. If you navigate to "Activity" and then "Account statement", you will see the reason for the transaction in the description.
In short, these transactions are not 'real' transactions, but rather administrative bookings.
If you want to find out more about the specific corporate action, you can usually find the necessary information on the (news page of the) company's investor relations website.