Cube Error Partition

Hello,

Errors in the OLAP storage engine: An error occurred while processing the 'MyPartition' partition of the 'MyMeasureGroup' measure group for the 'MyCube' cube from the MyDatabase database.

Any times i have this error, but when i click in REPROCESS the error is solved, why ? 

Other problems occur and i click in button REPROCESS theproblems is solved, why ?

Thanks !