It was a small thing for a green baker. It was a last minute idea and I know there is something about the raisins all coming to the top and without checking to be sure, I felt like I had heard or read that covering with flour helps with even distribution. With that in mind, I added them loosely to the flour and added said flour to the batter. I don't know if mixing longer would have dealt with the issue but I will find out next time. I'll just look it up now.
I think when baking it's best to mix all your dry ingredients in one bowl, your wet in another, and then blend the moist into the dry. It probably would have been best to mix the raisins in the the finished batter/dough at the end before putting the mixture into the loaf pan and placing it in the oven, or even adding them in the pan and just letting them sink down.