Answer: About the only thing that will cause collapse of a cactus is root rot. You might want to dig down around the rootmass (or unpot the cactus) to see what's going on. Perhaps the soil isn't draining well. There's not much you can do to stop a root rot. It causes vascular collapse and eventually the entire plant is involved. Once you see the symptoms it is almost always too late. Wish I had better news for you!
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