The bearing is over lubricated. The oil level is too high or there is too much grease in the bearing. The lubricating oil is contaminated with water. The shaft is overloaded because the pump is operating off of the B.E.P., misalignment, unbalance, etc. There is too much axial thrust.
Oil sampling is always a good idea.
If the oil additives are still present and functioning. If the oil is carbonizing due to high temperature. If there are solids due to corrosion, bearing cage destruction, or some other reason. You can estimate where you are in respect to the B.E.P. and know if the shaft i deflecting, or is about to deflect. You can tell if the motor is close to an overload condition.