What is the criteria of google to increase and decrease page rank of websites? Is this lot of traffic or back links or any other?
No one knows the exact criteria what google considers for page rank increase and decrease. It is just an assumption that number of quality back links a website has, site history, site popularity, domain names...
no one know the algorithm Google use for PR but PR flows form one page to another page so try to make quality back links on PR pages.
Pagerank is just one of two hundred factors Google uses. Low pagerank sites outrank high pagerank sites all the time.
Page Rank is link analysis algorithm , traffic has nothing to do with page rank, increase in quality back links will help in increasing page rank.