The return of the auto-vacuum

Hello Everybody,

This is the scenario:

I have a postgres 8.4, debian packages installed from pgdg repository
and version equal to: 8.4.21-1.pgdg70+1

I have a GPS database that insert about 4 million records on some tables
every week, the database is partitioned per week, I do a manual vacuum
every week and when we start the week then we 'vacuum analyze' the last
week tables.

However sometimes we have autovacuum running on older tables and I don't
know why, something like this:

autovacuum: VACUUM partitions.device_event_2015w05 (to prevent wraparound)

Those tables are almost read only, they are just involved in select
queries, but the system sometimes autovacuum them.

Any guess on why is the autovacuum daemon starting the process on those
tables ? It doesn't matter if they have been vacuumend before and
nothing have changed on those tables ?

Sorry, I still don't understand how the 'prevent wraparound' works, I
thought that 'vacuum to prevent wraparound' only got fired on tables
that were highly modified.

Thanks.

--
Nestor.