When you look at the fact that the wing mount takes out the top skin of the boom and the gear opening takes out the bottom skin there is not a lot left to hold things together. I use carbon fiber strips across this type of construction. I just finished adding CF to the Z B-25 118" just behind the wing between the opening and the opening for the side gun windows. I also added some 1/8 A/C ply reinforcement to the inside of the fusealage on this plane. It all weighs about 6 oz all totaled so not much extra. You could probably add some 3 oz glass cloth instead and accomplish the same thing. While it looks ok on paper when you see only about 3 inches of construction on each side it is not much. Better safe than sorry I think. It's not hard to blend in the CF and it prevents this type of cracks.

I'd agree with the vibration causing this. I also add control surface balancing to all surfaces except flaps to prevent flutter, scale or not.