Simon,
The radius on the hole was as much as the blame as the diameter of the hole. Ideally the hole edge should have been sanded back to a radius of half the material thickness on the upper and lower sides of the hole.

The current hole has a sharp edge to the hole cut-out, this along with a small defect would have caused a stress concentration point. Suggest you drop a hole size and radius the hole with corrosion protection where you have removed the anodising.

Jon