SQL Server - Log file not shrinking in AlwaysOn group
The issue is resolved. Apparently one of programs connecting to same large DB had been running for more than 24-hours, seemed it was in "hung" state, had several open connections to secondary replica-DB caused blocking. I guess that was causing not completing log replication from primary to secondary, because when I ended those programs, immediately blocking were gone and Redo Queue Size started decreasing instead of previously increasing. After about an hour or so, log file in primary node showed more than 80% available free space.

dbcc opentran and sp_who2 didn't give that much details to track this issue. What helped was

select * from sys.sysprocesses where blocked&lt;&gt;0
dbcc inputbuffer(spid)


The first statement was to find out what was causing blocks, and second one to kill.
I am just sharing if it helps others in similar situation.

TLog backup is being taken for all user-Databases in every hour only on primary node as mentioned in my post. So far TLog physical file-size was small, wouldn't increase and most of the time had more than 90% available free space - which is what its supposed to be. So after the issue is resolved, only difference is file-size is very big which I understand because it kept on growing. Now I need to do a TLog shrink (we did before) and it should go back to initial size.

TLog back doesn't need to be taken on all replicas. Technically you can, but if you are taking on all replicas, in case of Database restoration you have to gather all TLog backups from all Nodes and restore them which may be complicated.