This clearly has nothing to do with human agency. These dispersal events happened long before humans were around.

One of the species on the Azores returned to Europe. It seems that having got there in the first instance a very long time ago, it relatively recently migrated back to Europe.

It must have gotten there, we think, on some sort of migrating bird.

Darwin stuck snails on ducks' feet and submerged them in seawater and found them to die quickly on exposure.

Land snails, which we normally think of as being rather slow moving, can actually disperse enormous distances by hitching rides on birds.

I think because they live in trees and are particularly sticky, they're prone to being carried by birds.

Trying to get one of those birds and the snails together is problematic. So I suspect that some type of wading bird, with a cargo of stowaway snails tucked into its feathers, was blown off course by a storm and deposited the snails on these islands.