I used to think that Isshin was a member of a noble family (most likely Shiba) of soul society who maybe was cast out because of some law. However I have been pondering the thought what if he was an experiment of Aizen. Aizen was able to make shinigami into hollows 100 years ago but had not perfected the experiment maybe he had when he made Isshin but Isshin escaped and found love in the human world and was hidden in a gigai thanks to Urahara.
Part of my theory stems from the fact that maybe its not the high spiritual energy but the form of energy Ichigo gives off that gives all of his friends powers. It is the reason for this energy that the town has become the area where the key to the king of soul society can be made.