DOM isn't a special tubing, it is mild steel electrically welded and drawn
over a mandrel to remove the inner seam, but it isn't special voodoo proof
tubing, just heavy wall, and subject to fatigue and cracking like any other
tubing.
You may of had a stress flex spot there that worked it to it's breaking point,
it looks to have a good amount of corrosion both inside and out, so I wonder
how long has it been in service and not protected with a coat of paint?
Now if you had these made out of chromoly steel tubing, that stuff is more
or less voodoo proof and would probably hold up better, but requires special
welding techniques to stress relieve the weld site, not as user friendly as DOM
Wild