Now that the party has been attacked by zombies of bandits they had fought before along with their former slain companion, they are of the mind that they have made an enemy with some resources and a determination to get revenge. That is part of what motivated them to get back on track with their original goal of exploring the island, surveying the existing baronies and leadership, and determining if there is room for others. They are also now a bit paranoid about what they may encounter along the way and who is behind this worrisome attack.
As a DM I like where this puts the players and their characters, and they like where it has put me. I now have to get back to reading up on the region they are traveling through and preparing for potential encounters. The party is again spending good sums of money and looking for ways to gain more treasure, a win for moving the campaign along.
Despite being a sandbox campaign, it is a living sandbox with the actions of the characters having effects on the soundings and the surroundings not being just static and passive, but instead active and reactive. There are plots and plans by npcs that the players can easily become a part of or influence and from the first session they have indeed done exactly that without knowing the bandits had allies.
It will be interesting to see how the game progesses now that the characters are on the move again.