The vast majority of human lifts that start from a belt or cramped shelf - in sorting, loading, and last mile. Humans will sometimes rotate the box to center the load over an edge or regrasp from the bottom - but any given box is definitely going to experience moments with no support from the bottom.

I'd think the chances would go up substantially after they are removed from the truck and broken out of the palette, and before they are unpacked and the contents distributed.