Even you're already on top you can just relax everyone is trying to replace your rankings. So it's normal to went up and down on SERP.
there is nothing wrong with it, it is normal, you can't expect to be stay in top, if someone stays in top forever, how will you expect to discover better sites? Indexing may/must change
Not really. I have one site which has been at the top of google since 2001 and a bunch of other sites which have been #1 - 5 in google for their best keyword and they have been there for years and years.
Maybe other websites are optimizing and they are trying to get ranking high as you was before. Simply keep on optimizing and get new backlinks to your site, as the SEO is a daily process.