Eppo, I think it's because you tend to sit down against the seat harness to handle power, rather than edging the board harder. You're actually resisting the pull of the kite with your hips, so you push them out and... pooh. That's what I noticed going from seat to waist...
I've had Dakine Fusions, replaced yearly since 2010 and they've varied in comfort... 2013 is comfy once it's broken in. I didn't know the 14 was out yet.
Also have a Renegade, which is quite comfy but I'm still getting used to it riding up etc - waist attributes. I've found that not wearing a t-shirt over the wetsuit, or any thing under the suit, makes sure it stays in place a lot better.
Better work out for the abs

and my back never hurts with it... which was why I got a seat in the first place - had a microdiscectomy in 2006.
Also have an OR Session II in the cupboard... but I need to lose more weight