I probably have joined this forum but wasn't active when the karma system was introduced on this forum, and I've never really bothered to try to find out it's history, but what I do think is that, it's very possible that some users have already got to the level of senior members through their activity score on the forum before the karma system was introduced, and with enough senior members and above rank, karma was easily spread across to other low ranking members at the time, which also helped them to rank up to senior levels and joining in the Karma distribution.
And to the second part of your question @op, Karma is not auto given, it is users like you and I that gives each other karma, be it positive karma or negative Karma, this is actually why it seems like for some users, it's more difficult to earn karma because of their posting style, users award positive Karma to posts they find interesting, useful or helpful to a good degree.
So, if you want to grow your positive Karma, only make good quality posts and Karma will surely flow your way.