This is an excellent question, and one to which doctors are very interested in knowing the answer!
When referring to ADD & Dyslexia, I always used to say that no one knows which came first, the chicken or the egg. In other words: who knows?
Some doctors have gone as far as to postulate that children may develop ADHD in order to “speed up” or “catch up”, since, as a dyslexic, he or she has “missed” things because of his or her reading difficulty and may therefore, feel the need to “make up” for what was “missed.”
It may be that we’ll never know, but that doesn’t mean that we should do nothing! The time to conquer these disabilities is now!