Google changes their eveluation from time to time. Great content, domain age and authority backlinks are great now but that might change next time Google updates PageRank.
Your website Page Rank is changed according to Google algorithm and it might be that you website has suffered some losses or might have lost some value in Google algorithm to make you come down from PR3 to PR2. Work hard on your backlinks from relevant sources and also pay attention to your content quality to increase your Page Rank again.
