Interesting situation.... I guess what's happened here is that no-one in either authority was concerned (or even aware) until the publicity started. TC then jumped in and took a quick look, decided "nothing to see here, move along". Then the TSB says "hey maybe there is".
If it turned out that for a silly example the aircraft had a runaway nose down pitch trim that the crew heroically overcame and didn't think worth mentioning (prob: 10-6) then I guess that TC would have to reconsider their "no further action" judgement! Re other more likely causes - it will be interesting to follow this little story.