It's the mast top bending to lee. I have the same effect on my F16 and even saw it on skiffs like the 49-er when the gust hit the top of the mast. This bending off makes the leech extremely loose in the upper part and it wobbles as a result. They are depowering these boats likely nobodies business at 2.5 mtr width and 10 mtr mast featuring 20 sq. mtr. mains I'm not surprised. Like I said I've seen it on my 2.5 mtr wide and 8.5 mtr by 15 sq. mtr. boat as well in high force 5's

Wouter