There are so many ways for paladins to become evil, (blackguards, oathbreaker, death knight, narzugon) that I speculate that paladin powers came from a source that was originally evil. Then through some tomfoolery, the powers of good appropriated the paladin for their ends. Now paladins are either struggling to kludge their class to run off a good power source or that they are all sleeper agents placed to go off where they can inflict the most harm.