What is the mystery behind the headache if someone is chronically constipated?

Not causal: I may not be understanding your true meaning in this question, but I would say that there is a "common denominator" here. Low water intake can cause both of these problems. It's not that the constipation "caused" the headache, per se. But that dehydration leads to both. Does that make sense? Each of these problems requires a specific evaluation and treatment, start with more water! Ask again!