It's because they are grown, Grown boars height especially larger ones (1 and 2 star) allow them to basically use the ledge of the longship as a stair, with babies it occurs much less often and is how you are able to get them vast distances over water, however is not 100% fall off proof, i would recommend to plan your route near the shore, if they fall off they will usually swim to land and you can put them back in.