Hibernate validation with Spring data rest repository


I am trying to perform some operation in Spring data events HandleBeforeCreate/ HandleBeforeSave. Looks like with Spring data rest repository, hibernate validation (validation added with annotations like NotBlank) happens after executing methods which are annotated with HandleBeforeCreate/HandleBeforeSave and if validations fail, it rollbacks the original transaction.
My question is why the validations happen before committing, shouldn’t it happen before even executing the save() method?