"We've had several days of rain produced by different hurricane reminants."

Drainage problems and too much rain are the most likely culprits. Mums don't like wet feet. They rot. Clay soils like you have in the South rot most mums unless they are really well amended and the mums are planted slightly high for good drainage.

Sounds like crown rot, check to make sure there is no mulch touching the crown of the plant.

Thanks for the info! The area where they're planted, after a heavy rain, has water standing nearby. I've also got pinestraw under the mums. I'll double check that no mulch is touching the crown and I'll try to amend the soil around them. I just love the great info I get here!