GE make MS 6001 B gas turbine of 36 MW rating found third stage bucket shroud tip damaged. What may be the reason for that?

what precaution can be taken to avoid that as the unit was in running condition & it found only after opening of turbine casing during HGPI?

 

Some more information would be useful. any pictures of bucket damage? How old is the machine? Fired Hrs. at overhaul? Any strange behaviour before shut-down? Any pre-shutdown data?

Any of the above would be useful, we don't have a crystal ball out here!