In a nutshell, they say they were focused on the fusion of the metal and overlooked the depth of penetration which did not extend to the root of the joint. Apparently the weld was "almost strong enough" and they got lucky when the ISO testing was carried out. They also say that during in house testing (after the failure happened) some steerers failed but some did not and concluded that sending just a single frame for testing was not enough. This time they're sending 3 frames.