What happened?

A little over a month ago nearly all of the succulents on my porch started showing signs of deterioration. It started with one pot at first—my most recently acquired echeveria. I watched in dismay as the problem spread to my other succulents over the course of the next two weeks. Leaves were falling off, and my previously fat little plants were shriveling up. After a thorough inspection, there were a few of the most common issues happening at the same time. Here’s why all of my succulents nearly died: