Site Ranking has gone to Page X

[dunhill]

My Site Ranking for my main keyword has gone to Page X from Page 3 or Page 4, this happened a few weeks ago and I came back to where I was.

The question is why does this seem to happen to my website? All of the others seem to be very stable where they are for the keyword. I added 80 pages or so recently over the last 2-3 weeks.

The other question I have is there is one site for the same keyword who seems to be improving month over month, he is now on page 2, is this based on backlinking or something else?

Thank you.