Hi,

It would be due to your on page optimization.

check your index page contents with title and descriptions.

If find any mismatch with tags and your page contents then he can avoid that page.

Thanks
Jeperry :)

Always, sudden huge changes on the page will certainly make the position of the site worse. That is not at all a problem.

But see that you page is SEO friendly. See that your page's title and description meta tags does contain the keywords which you are using. And see that the page's content is stuffed with at least 15% of keywords.

See that you have submitted your site's site map, sothat Google could identify Index page of your Website.

You need to rework on your index page Meta Tags, Title and Content, I guess.

Changing up the text on the index page and the title tag, checking your logs and see exactly what phrases people are currently using to find your site. Also, check the competition for the keyword phrases that you're tracking, and see what differences there are, see how you can work to improve the page.

Aside from on page optimization are you also building relevant links using your targeted keywords?

My suggest, u should add ur web sitemap.xml into webmaster.Then u can ping ur site through ping service like pingoat. I hope it's helpful.