Chris, we need more information. Where are you located, what is your climate like, what has recent weather been like, have you changed anything? I would suspect a possible insect problem such as root maggots if you haven't changed anything. Can you dig up a couple of them at the yellowing/just turning mushy stage and look along the bottom of the plant, where the root meets the rosette or along the roots, for things that look like small maggots (or any other type of bug?)?
