From the Dark
It is suggested that the birth of an aasimar is the result of celestial blood in the family. This has not been proven. Angels and other creatures of pure goodness work in strange ways and are not always trustworthy.
Because of their celestial heritage, aasimars are expected to work for the common good. And inevitably, that expectation means they are expected to go into the mists. The few who don’t are driven into hiding.