I would suspect the Micro USB connector inside the Istick is to blame for the incident. I took apart an Istick that wouldn't charge and sure enough the Micro USB connector was the culprit.
Micro USB has been a weak point of every device that uses it. Talk to cell phone repair shops and they'll tell you the same thing.
I'm not a fan of apple but look at the Iphone 5 & 6 charging cable, they got it right.
Adding to what others have said about safety, make sure when you plug the charging cable into the Istick that you dont force it in and make sure you see the charge indicator come on, don't just plug it in and walk away.
By the way a GFCI is there to prevent you from receiving a shock and it would have tripped if there was a short to ground in the adapter but it wouldn't detect problems on the output side of the adapter so it wouldn't have done anything to prevent what happened.