Ok very interesting. Someone suggested the harness line. I had one out for the first time that session but had set it too long and was not quite tuned right so didn’t manage to get much power through it. Also the hook kept on pulling the waist leash off so maybe some refinements.
My mantis’ leech is completely hooked. Interestingly the mantis profile looks much flatter than the matador and strike, but I think because it is old as you say the draft probably moved back, leech hooked and now pulls hard in the wrong direction.
Unfortunately the same session I slashed the leading edge climbing over those rocks and now due a repair (and maybe a sale). Winging is very expensive.
edit: turns out it is a Mantis v1 and I think I got ripped off too 
the v2 has harness line points, and I also discovered that I had mine attached too far back.