As there's never been any official explanation as to why the welds gave way under stress, all talk on the subject is of course purely speculative. I don't know exactly what went wrong and neither do you, but we can of course guess.
Just to point out the obvious, there are hundreds, thousands probably of different folding bike designs many of which share Tern's style and configuration of bike frame & fold/latch mechanism. Why then do we not see folders cracking by the tens of thousands worldwide ?
What single common denominator (can't think of a better word off hand) links all the frames that failed but doesn't cross over to those that didn't. If and when Tern come out of the closet and explain exactly what the cause of these failures was, people will keep on guessing.