I'm wondering if there was perhaps some hydraulic fluid leakage. There are some pics and a video just posted on Youtube by a user called High Flight that show the Hunter recovering from that final loop and flying away from from the camera, flaps partially deployed with a mist of some sort adjacent to the flap on the starboard wing. I did notice that the flaps were definitely partially deployed during the loop.
Whatever the cause, it's a tragedy for all concerned and while we all mourn the loss of life, I hope it doesn't prompt a knee-jerk reaction resulting in the ban of displaying certain types of classic aircraft.