Explanation/Reference:
You must take some precautions with bulk update operations. To paraphrase the JPA specification:
bulk updates bypass optimistic locking checks (so you must manually increment the version column and/or manually validate the version column if desired)
Note: The object's version attribute is automatically updated by the JPA provider, and should not normally be modified by the application. The one exception is if the application reads the object in one transaction, sends the object to a client, and updates/merges the object in another transaction. In this case the application must ensure that the original object version is used, otherwise any changes in between the read and write will not be detected.
Reference: updating multiple rows using JPA
Reference: Java Persistence/Locking
http://en.wikibooks.org/wiki/Java_Persistence/Locking