Hello
How big is database?
Please show result of this query: select * from pg_stat_activity where query like 'autovacuum%';
I think here is running antiwraparound autovacuum. In this case all is normal, antiwraparound will produce a lot of WAL
andthis is necessary to continue database working.
PS: please note postgresql 9.1 is EOL.
regards, Sergei