He is lucky it let go on the line and not halfway down the track.
I wonder how many passes it took before it gave up?
That could be due to any thing from :
an inexperienced welder
wrong filler metal/ shileding gas
Wrong heat range
Welded tabs & brackets to light with not enough contacct area and not full pen welded
trying to get a 125 volt mig welder to do the work a 230 volt machine was required for/
Or all of the above.
I have seen stuff break off like that when guys ground the welds down to make them look pretty and went as far as filling in with bondo so it would look REEEEEEEEL SMOOOOOOOOOOTH
Guy was lucky.