What causes rotor unbalance?

A rotor is said to be in balanced state when there is a uniform distribution of mass about the rotational axis. i.e., geometric center and mass center coincides with each other as shown in the figure below.

To understand this query, consider a centrifugal pump is in operation. Due to wear in the impeller, impeller may lose some of its mass that results in unbalance. Following represents the possible reasons.

1. Wear due to cavitation in the casing.
2. Erosion in the surface of impeller.
3. Corrosion in the impeller.
4. Accumulation of foreign articles between the vanes of impeller.
5. Eccentricity in the rotor.
6. Presence of manufacturing defects in the impeller. (i.e. pin hole porosity. Etc).