Denmark is the other story. They introduced border control unilaterally, even before Schengen countries reached the agreement on national border control. Their reason? Cross-border crime. But scratch the surface and you find the handiwork of the right-wing, populist Danish People’s Party, which is allied to the centre-right government. The party is the third largest in the parliament, meaning that it can push for tougher immigration laws, with the threat to block key legislation if they are not introduced. The remark of Pia Kjaersgaard, leader of the party after border control was reintroduced: “I have worked hard for this.”
As you probably know, right-wing, populist parties are on the rise in Europe. For more on them, see this BBC piece.