IME, that is from root damage. I usually over water for a period of time when this happens, and makes one of the lower most branches break. The lower, older branches correlate to the outer roots, the oldest roots. The root that can get too wet if over watered. I have had many discussions about this with many different experienced growers, and nobody can put their thumb on exactly what it is, but often we refer to this as cannabis canker. I often incorporate a watering schedule that allows the soil to dry out a little extra one day a week to help combat this issue. Knock on wood, no plants have done it to me this year.....yet.