It is depends on your submissions. Do not intermittent times to submit. And these are the possible ways to aware decreading page rank: Doorway pages, hidden text, multiple domain, buying link and etc.
There are lots of reasons why your sites decreasing. Just like using duplicate content on different names, using various cloaking techniques, working with link farms, or etc. these are the possibilities to make your site decrease. Not only that, but there are more.
Are You doing and following ethical methods only to promote your site.If you are following unethical methods means it will punish your site.
if u r some of back links are less quality and google drops and if u r rank decrease, u r site start decrease visits, all factor relate to page rank.