This is puzzling. Do you see anything on the leaves? Doesn’t look like insect damage or sun scald. If there is erratic weather from hot to cold or irregular watering, this could also lead to die-back of branches. And there was a heat wave this week. This website may help you figure it out. http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/tomato.html. You could also take a sample to a local nursery for identification of the problem. Also suggest you clear the bark from the main stem of the plant so that the bark is not directly up against the stem where excess moisture may build up leading to disease. Feed with a slow-release or organic fertilizer formulated for tomatoes and supplement with a liquid organic foliar feed such as liquid kelp. Tomatoes are heavy feeders.