Vikings may have been influenced by the Romans, but burials have followed remarkably similar patterns for thousands of years all over the world with no influence from outside:
The dead are often buried with items to help them in an afterlife - food, money or something to trade to help the journey.
With items that were significant to them or their role in their life - jewellery, weapons, tools.
And are often laid out straight in 'best' clothes, relatives hold vigil over the body until burial/cremation etc.
We also base burials on the burials we've seen - so is it strange that they build tombstones like the old (Roman) one? In the same way that Egyptian rulers wanted a pyramid like their predecessor?