In most cases, these jiras are unassigned because the contributor who
posted the patch are the first-timers and do not have the contributor role
in the jira. It's very common for those folks to get overlooked.
Hadoop PMCs, if you have the JIRA administrator permission, please help
grant contributor access to these contributors. You help keep the project
more friendly to new-comers.
You can do so by going to JIRA --> (upper right, click on the gear next to
your profile avatar) --> Projects --> click on the project (say Hadoop
HDFS) --> Roles --> View Project Roles --> Add users to a role --> add to
Contributor list, or if the Contributor list is full, add to Contributor1