Although we can't determine an exact cause from the photos, likely causes are drought stress or (and this can overlap with or be a consequence of that stress) attack by Peachtree Borer in the base of the affected branch. You can use the information on our Cherry Laurel diagnostic page to narrow-down which issue may be present. Once branch dieback is this advanced, nothing can be done for that individual stem except to prune it off; thicker branches will need to be cut with a pruning saw since pruners or loppers won't handle stems over a certain thickness. If you aren't doing so already, monitoring the plants for watering needs and soaking the root zone as needed can help to avoid future dieback damage.

Miri