When you opened your account with the original lending bank, you would have a signed a Credit Agreement or similar document.
Such an agreement or document generally allows a bank / lender to sell a customer account to a company such as Aktiv Kapital, together with all rights that existed under such an Agreement. Your lender was not required to give you advance notice of any sale.
In most cases your original lender will have sent you a letter explaining the transfer of your account to Aktiv Kapital. In addition, at the time of purchasing accounts, Aktiv Kapital sends every new customer a letter introducing the fact that the your account has been sold to us. That letter is sent to the last known customer address provided by the selling bank. If a customer moved and did not update the bank with a new address, that is possibly why the account has been regarded as delinquent, and sold on.