I had this issue with my vortex 3, 11 brake fires in 28 jumps, cutaway last one. It seems to be partially dependant on riser length and the reserve packjob as the toggle was getting pinched between the reserve tray and the external riser covers. This didn't happen on previous reserve packjobs due to less bulk in the ears of the molar bag (My Assumption).
My solution was for longer risers as this moved the toggle away from the pinch point but I wanted longer risers for swooping anyway. I haven't had any brakefires since.
I tested this on the ground after my reserve was repacked. This was with the internal riser covers used however I had brakefire without using them too. I personally do not use them any more and lay them flat as it looks nicer without them and the external riser covers cover sufficiently alone.
Video testing this on the ground after reserve repack: https://www.dropbox.com/s/o1kyb33min4286c/C0014.MP4?dl=0
Still showing pinch point: https://www.dropbox.com/s/aek5z9fymc5atxn/DSC09162.JPG?dl=0
My cutaway: https://www.dropbox.com/s/l0of0ihsc5o5t82/Brake%20Fire%20Cutaway.mp4?dl=0