Hmmm,
I'll pay more attention to what my hips are doing next time. I've never really noticed much fatigue out of the saddle when on the road but I've also never forced myself to stay up for too long on the road. I think part of it is being made to ride standing and having erg mode on. Usually the resistance is lower than I would do when standing so it feels like I'd spin out if I didn't control my cadence (which I think some of my fatigue comes from slowing down). From memory, I do feel like a lot of the strain is just keeping myself up while trying to maintain a certain cadence.