I knew I smelled a DSB thread....lol
The term "crash" is a pretty broad generalization. I believe the general idea behind a "crashing" DSB, is simply algae that is out of control - uncontrolable phosphates. If you search around, then you'll see people having issues of algae, but they are looking for band-aid remedies....what eats this hair algae, what critters will get rid of this, etc. As mentioned, proper husbandry is important, as is understanding how a DSB functions. There are a number of things that can be done to sustain the life of a SB, but that is a different thread. It isn't always the sand that is to blame, the live rock can also be a problem, too. What to look for....algae that won't go away --> then test for inorganic phosphates in the water column, then test about 1/2 inch into the sand for phosphates, on the live rock...you get the idea. Try to figure out where the phosphates are leaching from (if phosphates are the problem) and go from there.