I am not that familiar with the 139 but when most helicopters loose a T/R blade the TRGB attachment studs let go or pull through. This looks like a structural failure and as the TRGB and upper part of the pylon rolled off to one side the blades struck the fin causing one to depart the scene. From a witness stand point it would look like one blade departing before the poo hit the fan. I hope they find the cause soon before someone gets hurt.