Yes, the web site have lost some back links, that's the major reason I guess.
I have 2 new web sites got PR 1->2 with 40 back links.
1 web site PR 4->3, because they've changed site structure and some BLs gone.
1 web site PR did not changed.
1 web site PR 5->4, don't know why yet...I'll find, I'll tell ya....
As you see, PR is not so easy to crack down
