I’m on the 777 at the moment, which is a pretty stable aeroplane (especially the -300). Most people I fly with are smooth in pitch and roll but the odd few just can’t stop giving little sharp inputs all the time, which on a 200ft+ wingspan machine generates ripples that wang up and down the wing bouncing us all around but with little to no effect on the flightpath. Same for the elevator. I spend most of the time after takeoff thinking “please put the AP in, please...” as any turbulence is almost entirely self-generated.
I haven’t been able to find a common factor yet, although many have come off the FBW Airbus...