What causes radial thrust in a centrifugal pump?

When a centrifugal pump operates at its best efficiency point (BEP), the flow angles match with the impeller vane angles. At the BEP, the flow velocity in the volute is uniform around the volute (i.e., there is not acceleration of the fluid in the volute). Therefore, the pressure around the volute is uniform. The uniform pressure around the circumference results in a net zero radial force on the impeller (or the shaft). Note this is theoretical. In practice even at BEP, there is a small radial load because the pressure is not uniform.

If the pump is operated away from the BEP, the flow velocities in the volute are non-uniform which results in a non-uniform pressure field and a net radial load (can be obtained by integrating the pressure around the circumference of the volute or the impeller).