Apparently because he failed to keep his 'residency papers' in order. Although European citizens generally have the right to live in other countries it was common up until fairly recently to have to register in the country you had decided to settle in.
Resident registration is still required in at least some EU member states, even when one is a citizen of another EU member state. In Germany, all residents - including German nationals - have to register each change of address with the local authority/authorities.