Pinning it would have probably saved it. I have had this happen when there is not enough CA on the hinge and it has not wicked properly. I use a T-Pin, cut it off flush, then put a small piece of covering over it. THEN I seal the gap with clear Ultracote. Never had a problem since I did this. I lost a Sig Kadet Senior to aileron flutter, and the hingle looked just like that...Learned my lesson.