Anyone know about this,explain please with reason?

You achieve high positions in the search engine after work on SEO. When you stop this work, your competitors continue to do it and they will take your place. Search engines change ranking factors constantly. And if by the current factors you are the first, after the change you can fall sharply. You need to analyze the changes in positions in your niche. Seo should always work.

It is important to maintain your SEO, while you are slacking, others are finding a way to make it on the top.

All are doing SEO for getting good ranking on search engine. competitors will beat you and get the first position, if you stopped doing the SEO after getting Good Rankings.

good question but its important do regular seo. even though you are in good rank if you do seo regularly competetiors will not beat you. and you will remain in the same position.

good question . we have to do it regularly in order to get top position and also search engine alogorithms makes us to do regular seo.

Do you know the story of Rabbit and Turtle Race?

SEO is a race. When you stop running, your competotors will overtake you and you will lose your ranking. If your competitors are also running or walking, you can't stop. I work for a web design company in Madurai and the industry is very competitive here. So, we regularly do SEO for our website.
We will not stop our SEO efforts even if reach the first position for all keywords. The reason is as soon we stop doing it, our competotors behind us will overtake us. Sometimes, it can be very difficult to get back the top position once we lose it.
However, we have stopped doing SEO for one of clients after reaching the top position. The reaon was the industry of that client was not that much competitive so it was unlikely that the competitors overtake our client.

So, it is all depend on the competitiveness.

The competitors are spreaded over every niche and they wait for an opportunity to grab Google SERPS.

You don't have to do SEO if you have valuable content. Bad content requires consistent SEO, while great content markets it self.

When I say great content I mean great user experience. Was the user satisfied with the page he or she visited? Did they get what they want? Are they coming back? These are all things you should consider.

And no, you don't need to do SEO continuasely.

I heard a great quote from Income School channel on YouTube ...

If you are gonna put hours of work into SEO. You would be better off optimizing and improving your content.

You know, links aren't everything to Google. They look into user behavior too.

Just my 2 cents. :)

commented: "Bad content requires consistent SEO" +15

Because the competition is too high,if you will not keep on doing wok on it , it will automatically goes down day by day.