We have seen this happen on occasion, even in nursery pots at garden centers, though don't have a concise diagnosis as several factors could be responsible in creating similar or overlapping symptoms. Root rot, an infection like one of the wilt diseases (Verticillium or others), and Botryosphaeria canker could be possibilities. You can look around the crown (where growth emerges at the soil line) for vole gnawing, though we doubt chipmunks will be bothering it.

Unfortunately nothing can save the pictured plant and it needs to be removed. We recommend considering a different species replacement, since Buddleia can be invasive. (Some cultivars are supposedly sterile, though we don't know how reliable that trait is from one cultivar to another.) If you replant Buddleia, make sure the site has well-drained soil, and the plant is only watered once the soil about six inches deep becomes somewhat dry to the touch at that depth.

Miri

Thanks so much for the information and prompt reply. I did a lot of research before choosing my plants, but had no idea that buddleia was considered invasive!

You're welcome.
Most cultivars of Buddleia do set viable seed, though over time, more cultivars introduced into the market are supposedly sterile (or nearly so), in addition to having traits like a dwarf growth habit or larger bloom clusters.

Miri