I am going to guess too much water caused root rot. This is only because you said they had plenty of water and sun and this plant does not like too much water as it is sensitive to rot. I also see the mulch around the plant and this could hide the moist soil to contribute to overwatering.  It could also be a viral or bacterial infection or root damage from vole or mole. Dig in and feel the soil. If the soil is moist then I place my bet on too much water. Don't feel bad as you learned something and ornamental peppers are known to be a bit challenging to keep happy. Keeping them in a pot helps with perfect drainage. Keep growing, Marianne Binetti