The lower search results usually fluctuate much more than the top search results. If your website is new in the top 5 results, you can expect more fluctuation than websites that have been listed in the top 5 results for a longer time.

You can also expect higher fluctuation if the keywords are related to a hot topic. If Google shows results that are marked with "two hours ago", or "one day ago" then it's likely that the keyword is related to such a hot topic.

If your website was ranked in the same positions for several weeks before the rankings dropped, you should take a closer look at the changes.