Looking at the pictures and remembering back to when I was welding, thin and heat treated steel you have to be careful when you weld to not get the surrounding area too hot. Welding by hand you would tack weld say every inch then come back and weld one inch section on one side then move to the to another area and weld a section, moving around so as not to get one area too hot. Welding too much at once can weaken the metal around the weld and make it break right next to the weld like that. so its plausible from the pictures there could be a welding problem.