vacuumdb idle processes

I have done a setup to run vacuumdb with 8 parallel jobs daily. It has been running quite well.

Just today I saw there is an aggressive autovacuum process running(to prevent wraparound) on one of the table. vacuumdb which started later spawned the 8 connections. One connection (doing vacuum on the table on which an aggressive autovacuum is running) is waiting for "autovacuum(to prevent wraparound)" to complete while the other 7 connections are just sitting idle.

I am okay that one connection is waiting since an aggressive autovacuum is running on that table but how come other connections have not released the sesion yet? Any reason for this?

Thanks,

Nikhil