When using the sledge/pliers on sand, the new block still behaves as if physics was enabled. With the sledge, the block removed has no physics applied, as is visible by the fact that it will not force sand above it to fall down.

https://bukkit.atlassian.net/browse/BUKKIT-1312

Apparently this is a known Bukkit bug, hopefully will be resolved.

There is a way to cancel the fall event, the question is whether there is an efficient way to only do it after a Sledge/Pliers use.

    @EventHandler
    public void catchEntChangeBlock(EntityChangeBlockEvent event) {
        if (event.getEntityType().equals(EntityType.FALLING_BLOCK)) {
            event.setCancelled(true);
        }
    }