ES80 with loads of memory paging out processes

My basic question is: Why do processes page out if there is plenty of
free memory?

Detailed description of problem:

I have a new ES80 with Tru64 Unix V5.1B that has 32Gb of RAM. I'm only
using about 12Gb for my database application, so I was surprised to see
that, on a fairly regular basis, the process is getting paged out. My
definition of "paged out" is that the process appears in the ps listing
with square brackets - i.e. [process_name] instead of process_name. I
pick this up because I have a monitoring script I wrote that checks to
see if the process exists and notify me - I beefed it up a bit to notify
me when the process has brackets around it.

I was doubly surprised to see that this is happening even when the
application is doing something. Typically it is running a very
long-running database query.

Why is Tru64 Unix doing this? Could it be a performance problem (i.e.
slowing down the application)? I assume the "sticky bit" is the way to
fix this phenomenon?

Much thanks!