1) Links may disappear. You have no idea how many sites that were online 4 or 5 years ago have expired domains / unpaid hosting / etc. today.
2) As more and more "link juice" is pumped into the web, the value of previous links is worth less and less. If there were 1 trillion pages five years ago and there are 1.2 trillion pages today, that's so many more links pointing all over the place. If a page had 5 links five years ago and those same 5 links today, and hasn't gained any new links, that page would be worth less in today's standards.
I got warning message in my Google webmaster tool (Manual Action):
Google detected a pattern of unnatural, artificial, deceptive, or manipulative outbound links on pages on this site. This may be the result of selling links that pass PageRank or participating in link schemes.
My PR was drop dramatically. Any better option how to fix this issue?
Google page rank can fall down once gained depending on various criteria. The content of your website is very important. The article should be unique and of good quality related to the niche of your website. It also depends on your website backlinkings. Unwanted spam linking causes your website to lose page rank and even get banned. So once you gain page rank for your website, keep a tract of your content and link building to maintain that particular page rank and even improve it in the next update.
And Googles algorithms also changes. I have sites that havent been changed for years and they have jumped from PR0 to PR4 and back again a few times...