These probably passed through at least one middleman. Like everyone, the Vikings liked high-status items. They probably were acquired by trade, perhaps in Constantinople, where Vikings were a somewhat familiar site during the Middle Ages (9th century onward). May have been bought in the markets there, may have been given as rewards for service to an Emperor.
There’s no evidence that these were taken as booty, but they could have been taken by one Viking from another after they picked it up somewhere. Obviously there’s no documentary evidence for the origin of the hoard, if there were, the hoard would have been looked for and recovered long ago.