350X cracks in frame,,,

[86350X]

I know it is very common to have cracks in the frame at the back near the seat grommets, What causes these,,,, Are there any other places where cracks are common? What should I lok out for, How can they be prevented? I just bought a new MIG welder, so I will repair the ones in the back,,

[hrc200x]

look above the left foot peg where the motor mount attaches to the vertical frame, kind of up by the carb, usually they start cracking on the inside so its hard to see. I think the cracks back by the grab bar and seat latch are from either wheeling over onto the grab bar many times, or maybe landing hard off a jump and you butt lands on the seat hard???

[Tecate performance]

also check the front downtube below the steering head gusset