I concur with sycamore's observation. I could be a combination of the torque (such as it is for an O-235) and latent disturbed air from the 737. Always consider the direction of any wind on the airfield, and where it might blow turbulence from another aircraft. After allowing adequate time for turbulence to dissipate, still fly toward the upwind area of possible turbulence whenever possible.
Though yaw is possible when encountering changing airflow, if full rudder applied and held will not control the plane at 40 knots, you encountered air which would have been much better avoided.