Cold be a sharp edge on the fitting...

Could be that you are overstressing the wires with the downforce on the tailplanes. If its this one... where's the CG? How fast are you flying? How "hard" are you pulling for the loops?
Pulling too hard for loops at high speed will put a HUGE stress on the tailplanes and they can try to flex... and thus snap the wires (which would mean the wires saved the airplane... by taking some load and preventing the tailplanes from snapping.)

What material are you using for the flying wires?