At first I thought it was just physical damage of some kind (though I can't imagine how that would have happened) but now I'm noticing these spots spreading to other nearby succulents. As they just sit quietly on their shelf and are never moved around or disturbed much I'm 100% positive this isn't just damage.

They are hard, dry brown spots. I've checked out a few webpages for succulent health problems but I can't seem to figure out what the issue is. Anyone here know? I would greatly appreciate some help with this.

Here is a link that might be useful: More pics here.