Barefoot, the stack of cogs in your cassette is just riveted together. If the setup hase been loose like this for some time and a bunch of miles the rivets will have stretched and worn loose. Now all the parts are sort of floating and relying on the splines to hold things together. Why some move together and others don't may well be due to the way the rivets wore or broke. Also both the 12 and 14 tooth cogs are actually loose cogs and only the rest are riveted together. This likely has something to do with what you're seeing.