DSB question

[daplatapus]

Yesterday I did a water change in my system. 77 gal display and 80 gal sump. In my sump, I have a refugium with a DSB. Over the last couple months I've noticed strands of chaeto laying on the sand and thought I'd clean it up a bit. So I took my siphon tube and ran it lightly across the sand in the hopes of just vacuuming it all up. I noticed though in one corner of the refugium that the gravel had turned into almost solid rock. I understand the dangers of screwing around too much with a DSB but with utmost caution I jabbed it a bit to break it up back to sand again. Does anyone know why this happened? I'd assume when it gets like that water can no longer sift and flow through it then I'm really in trouble.

What causes sand to stiffen up like this and should I be getting little critters to mix the surface a bit? I've read that is a bad idea but I can't think of how to prevent this from occurring again.

Any help would be most appreciated