When is a shared library unloaded?

I have a shared library loaded into postgresql using

CREATE FUNCTION medbevent() RETURNS trigger as
'/usr/lib/pgsql/libmedbevent.so' language 'C';

I'm randomly having issues with my triggers not firing (it turns out
they are getting removed) and when i turned INFO logs on i saw the
following

2007-08-14 13:41:44.740305500 LOCATION:  _fini, medbevent_init.c:129
2007-08-14 13:41:45.790850500 INFO:  00000: medbeventlib -Unloading
library _fini() is called

Could someone please explain when a shared library gets
loaded/unloaded and how this all works?  I'm getting the above logs
when i wouldn't expect to see the library be unloaded which is causing
me problems.  Could the library get unloaded in this manner due to an
internal error in the library?

Thanks,

Jon.