If for any reason you cannot wait until the active task to finish, you can always do:
sudo systemctl restart rpm-ostreed
To regain control.
If you want what rpm-ostreed is busy with, you can
sudo journalctl -ru rpm-ostreed
From journal, you can find out what the initiator is. Like:
client(id:gnome-software dbus:1.113 unit:app-gnome-gnome\x2dsoftware\x2dservice-1950.scope uid:1000) added; new total=1
Allowing active client :1.113 (uid 1000)
Sometimes, the mirror site chosen for the transaction is slow, or the transaction data volume is huge, so it might take longer to finish.
Sometimes, you might want to manually trigger the transaction in foreground to see what is happening by:
rpm-ostree update --check