MikeyMK

I've seen a 90's GT rear frame cracked around both rear brake mounts. 7005, thin wall.

The one in the picture is missing a vital brace link between the seat stay and chain stay. So fatigue was inevitably going into the metal on braking alone, without even seeing the frame design. The weld is stronger than the tube because it's thicker, the tube spreads the stress everywhere along it except where it meets the weld, so metal will almost always fatigue/crack at the edge of a weld, where flex meets rigidity.

Absolutely no surprise this frame has snapped there. One will have cracked first, the rider not detected it, then when the other cracked they both went completely.