How is the job processing order set?

Scenario: BE has lots of jobs queued that are supposedly "running" with the green arrows but are actually doing nothing. One job (a duplicate to cloud) has "loading media" as the status. I kill that job and all the others start running as expected - they were basically being held up by this one stuck job.

These other jobs are for different severs, backing up, duplicating to/from different disk, cloud and tape storage devices - so why are they held up by a totally unrelated job? And what can i do to stop that happening?