Why might the Shetland Islands share some cultural features with Scandinavia?

Because they were colonized by Scandinavians

Explanation:

One of the main reason is that many Viking groups from Scandinavia. From the 9th century, Norsemen settled in Shetland. They set up their own civilization. They were later invaded by Harald Finehair, the first king of Norway and remained under the control of Norway. The Scots on claimed sovereignty in the Shetlands in 1223. Over the centuries many of the traditions, languages, music and heritage in the Shetlands was closely linked to their Scandinavian background.