Here is one reasonable answer:
”
“A lot of people have speculated as to just why the movie has the Vikings speak with Scottish accents. The reason is because the audiobook, released in 2004, is read by acclaimed actor David Tennant, who is Scottish. The way he voices several characters is quite similar to the film, notably with Gobber and Stoick.”
http://www.youtube.com/watch?v=Dh28W9Lqw2w
@Imadethisupwithnoforethought.
“The use of Scotch to mean ‘of or relating to Scotland or its people’ is disliked by many Scottish people and is now uncommon in modern English. It survives in a number of fixed expressions, such as Scotch broth and Scotch whisky.” Dictionary.com