Originally Posted by joejack951

Maybe not all the cogs, but at least the three big cogs on my Ultegra 10 cassette are pinned together (can't remember if it was 3 or 4). Makes sense since for a given chain tension, those cogs exert a much higher force on the freehub than the little ones. All cogs exert the same force; they are transmitting it to the hub shell, which is the same size for each cog.

I haven't been working on bikes long enough to say this with any certainty, but I would think digging like shown in that picture would only occur if the cassette was not properly tightened onto the freehub (too little torque).Not the only reason, but definitely a good one.