Why auto vacuum almost running all the time on one toast table?

v9.1.3 on Ubuntu 10.04 server. I have one table which has frequent
insert and weekly deletion, no update. Recently, I found the auto vacuum
on the toast table of it almost running all the time. each run took
around 1 hour. I don't think there are so many inerts so that it reaches
the auto vacuum criteria(20%? changes). And there is no such frequent
auto vacuum on the main table. The last auto vacuum of the main table is
on 3rd Jun. what might cause this behavior?

# grep pg_toast_16922 postgresql-2012-06-12_000000.log
2012-06-12 01:22:56 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"
2012-06-12 04:27:49 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"
2012-06-12 06:58:02 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"
2012-06-12 09:27:38 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"
2012-06-12 12:17:33 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"
2012-06-12 15:05:31 CST [@] CONTEXT:  automatic vacuum of table
"oodb.pg_toast.pg_toast_16922"