What causes this and how do you prevent this from happening?

Sounds like overwatering or poor soil drainage.

Drought tolerant plants need to be planted & watered carefully. If they are planted too low in the ground (covering 'the crotch') they can rot. Often, they are planted correctly but an overzealous gardener waters too heavily and moves the surrounding soil so that it (or the mulch) gets too close to the base of the plant. Same result, it causes rot. We need to learn how to care for these new specimens… at the nursery, we call it "killing them with kindness", because people mean well, they are just uncomfortable with letting them dry out. For now, use a water meter to check the soil around your plants until you are used to their needs.