Likewise. My first thought was rapid-rolling and roll-yaw coupling. Did the aircraft in question have the fuses which restrict the roll-rate? However there is no rapid roll evident in the video. It doesn't appear to be flying that slowly so flick/spin looks unlikely. Next thought was the training and experience of the pilot. I see from the Gnat Display Team website that he was in the RAF from 1998-2001. Three years. Not even long enough to become operational on a FJ type. However one of the team pilots has plenty of street cred so it can be presumed that he will have passed on his expertise. We'll just have to wait and see what the AAIB comes up with.
RIP.