Even from that photo you can see that there is some slow crack propagation followed by fast fracture cracking.
And the point where the cracking started is right at the stress concentration point. If you get right in close and look at the metal it will appear dull and possibly ringed at the bit with the paint off and long bright strips in the other fast fracture bits.
Any materials engineer will take less than thirty seconds to tell them its a fatigue failure. If it was done by miss handling I would expect to see plastic deformation of the surrounding material. And as I presume its made out of 3mm alloy you could hanging the plane off a strip of it with out braking it. If you had pushed and pulled it it would have broken at mounting points.
You need to look up words like ductile tearing and fast fracture.