Going by the create index documentation, parallel degree can only be set to some integer value by user which defaults to 1 while using NOPARALLEL clause.
Contradiction is, for 3 of indexes the Degree is set to "DEFAULT". Which process set it!
One exception I observed is the auto task "auto optimizer stats collection" was running and DBA bounced the DB. As a result of this the particular day job terminated abruptly with below error.
dba_autotask_job_history
dba_scheduler_job_run_details
My doubt is, did this job (dbms_stats.gather_database_stats_job_proc) set the degree to "DEFAULT" (to gather statistics in parallel) for those 3 indexes and did not revert back due to DB bounce! Should there be any other factor to look for this index degree setting, please advise.