Why did Denmark blockade German Baltic and North Sea ports?

During the German-Danish War (also known as the Second Schleswig War), Denmark implemented a naval blockade of German North Sea ports as a strategic military measure to exert pressure on the joint Prussian-Austrian forces.1

The blockade was the most effective effort undertaken by the Danish navy during the conflict. By severely threatening German shipping, the blockade forced many German vessels to reflag as Russian ships in an attempt to evade interception by the Danish fleet. This economic and logistical disruption was significant enough that it prompted the Prussian leadership to persuade the Austrians to intervene and attempt to break the blockade, ultimately leading to the Battle of Heligoland.1