Welcome to the Q&A Forum

Well....there are so so many things that can contribute to a fall in rankings...that I can't mention them all here... But a drop - if I read you right - from #6 to #10 is not a drop really at all...just normal fluctuations...i.e. this is how the rankings flutter... Have you checked tho to see if you're looking at the rankings using Personalized Search? Incognito? WebHistory on/off? Do some googling for those topics....read...and learn! Not trying to be dismissive here - just that there really is no 'answer' that you'd be satisfied with....honestly!

If you are moving down consistently then it is most likely not all of your competition suddenly started to work for that keyword.  How to fix your site is complex but you should start with looking at the page that was receiving the traffic and check for changes that were made recently, make sure that you are following best practices for your content and technical pieces.  You should also check your back links as recently many people who had used certain link building tactics have seen decreases as Google updates.