Certified GKH Gardening Expert
It sounds like it is developing a root rot. Typically, root rots are brought on by overwatering. This plant enjoys dry conditions and will be more susceptible to root rot if placed in a place with wetter condition. If the location they are at does not dry out well, you may want to consider moving them.
Root rot can also be caused by a fungus. If you feel the location is dry enough for these plants and that they have not been overwatered, you can try treating the soil with a fungicide. Fungal root rot is difficult to control sometimes, so there is no guarantee that the fungicide will work, but there is not much else you can do for them at that point in time.