In fact Bremen (by far) was the most common port of departure for people from old Austria-Hungary. The port of departure largely depended on the steamship ticket agent operating in a give area. Leaving from Bremen was a shorter journey.
In regards to the choice of Dragutin for travel. I could not tell you why but the names Karlo and Dragutin are interchangeable. Have you seen the actual parish baptismal record showing his name.