I have had this happen too, since I grow indoors in my basement where it can become and stay colder I believe Hackerman is correct when he states lower temps. Can contribute or even be the cause itself.
Dont worry about it,
In fact, to expand on the subject, I have noticed the SAME strain plant do this when its sisters were not, so, this leads me to belive that genetics in part play a role.
Have you ever cloned a plant and have some do great and others flop?
I have had the same strain seeds and clones act differently at the same age and size, some burn easier, and some tolerate and even like more nutes, even though their technically twins, genetic sisters.
No different than humans, were the same but we each have own own tolerances, strengths and weaknesses.
While this can show up in certain plants as a phenotype expression, it can also be a deficiency. Are you in soil and using natural nutrient sources (also known as organic) if so then you need to get some "dolomite lime" it has magnesium which will help correct this issue. Just add about 2-3Tbsp(to 1-3gal soil) to the top of the soil and mix it into the soil as much as possible before watering it in (right before a watering). This will also help to maintain a balanced pH in the soil.
If you are in larger containers of soil then you will need to add 4-6Tbsp and then do it every 2-3 weeks as you notice the leaves begin to show signs of the redness. The best way to do it is to mix in the lime before planting so that it is mixed throughout the soil. I am not sure what the ratio is when doing this but you can find it in the organics section under the super soil recipes