On other cause of headaches after biking is from the neck. Bending your head back contracts the suboccipital muscles or muscles at the base of the skull. There are four nerves in the area called occipital nerves which runs from the back of the head to behind the eyes which can sometimes get irritated. I had this everytime I went scuba diving. I started stretching, had a massage and got my neck manipulated and usually that gets me six months down the road. This probably isn't as common as electrolyte imbalance though.