|MadSci Network: Botany|
First of all, I want to apologise for taking so long to respond. I had to visit the university library to answer this one as I have no personal experience with growing buckwheat.
The fact that buckwheat turns red should not be so surprising because many members of the buckwheat family, such a rhubarb, turn quite red. The red colour in nearly all plants is due to the production of anthocyanins. A smaller amount of anthocyanins will result in brown. A larger amount will result in a redder colour. So the buckwheat that grew in your garden turned red because it was producing more anthocyanins.
Why might it do that? Well you could have been using a new variety that naturally produced more red pigment. Also, research done on Buckwheat back in 1907 by Linsbauer showed that the amount of red pigment produced varied with the the strength of the light and the time exposed. Did you plant it on exactly the same date as usual?
Now, I have not been able to exactly answer your question, but I hope that I have given you some ideas to consider. If you have any more questions, please do not hesitate to contact MadSci Network. Next year, you could do a project on this by planting some buckwheat on different dates and observing the effects.
Try the links in the MadSci Library for more information on Botany.