Answer
To help fight tax evasion and to protect the integrity of tax systems, governments around the world introduced a system for automatic exchange of information on financial accounts between countries. This system is known as CRS and is incorporated in Dutch law.
As a result, DEGIRO is obliged to verify in which country a client is (potentially) a tax resident. DEGIRO is obliged to share this information with the Dutch tax authorities. Subsequently, the Dutch tax authorities share this information with the tax authorities in the country where the relevant client (potentially) is a tax resident.