Don't know why, either. Most people who suffer from chronic anxiety have no idea why they have it. Some people can trace it to some traumatic or dramatic event in their lives, but most can't. Science hasn't found a cause, either. But one thing psychologist seems to agree on is that childhood fears aren't the same as adult anxiety. One probably doesn't lead to the other.
If you've found the way to calm yourself, there's no need to torture your brain with trying to discover the reason why that happened in the first place. You should be grateful and proud of yourself for finding the way to cope with it. Not many people know how to achieve that. If you feed your brain with negative thoughts you can't expect to be happy, and I kinda got the impression that you are aware of that. So, once again, forget the past and focus on the present moment, that's my advice. I wish you all the best.