The other reasons for paging are that the operating system is retrieving new data or software from the disk or writing data to a data file (not the swap partition).
You could use the top utility for a quick look at what the operating system is doing.
I recommend that you run sar. It is in the sysstats package. You can run sar against the running system. If you do this it is like running vmstats and iostats at the same time, more or less. Sar is more useful if you run its data collector in the background and then use sar to create reports based on that data.
You have to configure sar. In my distributions it is installed in completely useless mode. You have to edit the configuration file and the cron jobs to get the data collector to start. Once the data collector is running automatically it will record system activity and resource usage. Let it run for a few minutes or an hour when the system is paging heavily. Then get a few reports from sar. You pass parameters to it to tell it what parts of the system you are interesting in investigating.
Once sar is installed you can find more specific information by using the man pages.
The sar utility can make more in depth reports than top but it takes a little bit of time to collect the data.