Hi

I am new to SEO Powersuite.

One of my sites has a PR3 for homepage and 10 pages on PR2.

I recently ran Website Auditor and it found the PR2 pages are now PR0.
I checked this manually and it is correct.

How can this have happened ?
The only change if any would be the keywords and meta description added to all pages (not just these).

Any ideas ?

Sorry you've experienced this. Could you please give us the URLs of these pages? Changing keyword densities on the page is very unlikely to cause that (unless you really 'spam' the page with keywords). Most often PR loss penalty is inflicted on sites for some linking activities, like selling links or complete stop of backlink growth.