Actually, as far as I know, most hoards in Britain were of Viking origin, and were offerings to the gods.

The reason why they are theorizing that this horde is not an offering is that the vikings very rarely showed such generosity towards their gods. There was frankly too much gold in the hoard.

What is quite out of the question is that this hoard had anything to do with the Romans leaving. The gold has written inscriptions tentatively dated to the seventh to early ninth century.

See Wikipedia for more info: http://en.wikipedia.org/wiki/Staffordshire_Hoard