What wheels do you have on it? I've found the stock Roval wheels to have quite a lot of flex. Going back to back with my carbon wheels the difference in steering precision and rear end stiffness is substantial.
Nukeproof Horizon V2, I always thought they were reasonably stiff really as I've got three sets on 3 bikes, much better than the rovals. May double check the spokes but it feels like the suspension linkage to me. Or maybe just the weight of the ebike