Like said, in most cases it's random. Can be PH, cold weather, temperature changes.. but i have to say, this happened to me one time, with an excess of light at some point of the colas, with "not appropriate" LED lenses, was pre-installed with the equipment. So, that doesn't seems like that in your case, but for me, the solution was to put a little distance from the LED, and delete this "bad" penetration lenses who was included in the device.
Good luck for your grow.
420