Local jamming issue in pump shaft rotation

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It is a double-suction, single-stage pump. After major repairs to the pump section, testing showed normal operation (the motor was not repaired). The vibration at both ends of the pump was no more than 2.2 mm/S, while the vibration at the motor drive end was 0.9 mm/S. The radial deviation of the coupling alignment was around 0.20 mm, and the axial deviation was within 0.10 mm. The pin coupling is not used frequently; it operates once every two weeks for about 10 minutes each time. During the most recent operation, it was found that the motor could not rotate the pump. After removing the coupling pins and attempting to rotate the pump shaft, it was discovered that certain parts of the shaft were stuck tightly, while other parts moved easily (this phenomenon did not occur during previous alignment and rotation attempts, when everything worked normally). After loosening the fastening bolts of the bearing housings at both ends of the pump, the pump shaft rotates smoothly and evenly without any jamming. I would like to know what causes the pump shaft to stick. Have anyone encountered similar issues? How can we prevent this from happening again in the future? Thank you!