Salesforce Exam CRT-251 Topic 7 Question 84 Discussion

The most likely cause of the records failing is that updates to child records that have the same parent record are being processed simultaneously. This can cause the records to fail because child records must be imported after the parent records have been imported, and if the parent record is being updated at the same time as the child records, the import will fail. The other options are not likely to be the cause of the failed import.