Aren't the side branches too small for a plan...

If she's spending a great deal of time in the shade, she may be focusing her energy on growing upwards to try and break into the sun light. Plants often respond this way to competition. Even if she doesn't need more, she will fight to make sure that remains being the case. Most cannabis plants want at least 8 hours of direct sunlight to be happy outdoors, but I bet some others thrive regardless. That's all I can think of, your strain is indica heavy so while it could just be exercising its sativa side, I doubt it.