Dixierose,
First thought would be a mole tunneling under them. But, you surely would spot that problem although it often happens unnoticed.

Second thought...remove several of the thickest stems, split them open lengthwise and check to see if there are any tiny 'ants' living inside the stems. This is a new problem we here in the south need to be alerted to. Reports are trickling in that plants are suddenly wilting due to Formosan termite living inside the stems. Doubtful that this is your problem but they are in the Charleston and south areas.

Next process would be to pull up a plant and check the roots for signs of root-knot nematodes. Another possibility. And, it could just be a weather related problem. We have had lots of rain followed by blistering heat which may affect mum growth.