Does your site have duplicate content? If so remove the dup content add a few dofollow backlinks and you should be back to where you were.

The general theory is you probably backlinked too much at once.

But, it could be just some natural Google Dance.

Note that when you backlink to a site that has not been backlinked for quite some time, your site may start to bounce around (like your site). If this happens, just keep backlinking.

After the bouncing, it should return to a better or same position.

This was my experience. I hope it was useful to you.