I'm going with nutrients. The more I can give my plants, the fewer purple stems.
My plants get plenty of calmag but can still get a few purples. I can't go with light burn because I've gotten them on the lower branches. Just go with it. There's a good chance you will have yellowing, orange-ing, and other pretty colored leaves when your plants get in their final weeks of flowering. It's nothing to worry about, either.