There is not enough information to form an answer here. I am going to make some assumptions based upon my experience. Some of this may be obvious stuff, so bear with me, it deserves repeating.
Google selects web pages to display in the SERPs that closely match the users query. Search trends are constantly changing and can change from hourly to daily. Sometimes, a new popular search results in web pages, that while appear to match the users search query, does not address the question the user has. That is because new questions arise everyday. 15% of Google's daily queries are searches never performed before. That says a lot! Each search is essentially a question and each SERP impression a potential answer. It is up to the user to decide if the SERP impression appears to answer their question. Google tries it's best to match the results correctly and uses successes (good CTR) and failures (bad CTR) to adjust how it tries to answer the question in the future.
I see shifts in Google Webmaster Tools where impressions for certain pages and search terms shift to the top but the normal click-through rate does not change or changes little. That is, if a page normally performs at a certain level but then suddenly shifts to the top of the SERPs, the page performance may not change. This is actually normal. Sometimes, however, a page can suddenly rise to the top of the SERPs and the click-through rate sky-rockets. It seems to be one or the other.
Here is what is going on. Google is trying to answer the users search (question) with an impression of your page (possible answer), but your page and impression does not seem to answer the question. And your page may not. Or it may. This is where the ever-evolving work of SEO comes in. I am constantly checking for pages that have increased impressions and lower click-through rates and I try and research what the user may have wanted. If it is appropriate, I adjust the page to better answer their question and make it appear in the SERPs as though it is the best possible page for the query. I learn a lot that way. Otherwise, I adjust the page as not to mislead Google which is easy to do accidentally.
To answer your question about a traffic drop. Sometimes that is the way it goes. It is like fishing. Sometimes the fish are there and sometimes the are biting, sometimes they are not. Because search trends shift and can shift rather radically very quickly, I have seen deserts in search results as much as I have seen Shangri la. The goal is to answer as many user questions (searches) as you can. But you have to accept that not all questions do you have an answer for. Take this opportunity to broaden your content within scope and search for new ways to answer your audiences questions and perhaps expand your audience a bit.