In the last months I noted that one of my main keywords (website section) was dropping quickly, did I do maybe something wrong? For instance to add a menu entry and a h1 with the exact keyword which is actually the real name of the section?
It's difficult to tell based solely on the information given.
More likely it's Google's recent spurt of updates. However, based on what I see, you have only occasional click through by users in spite of a fairly consistent impression rate and positioning. It's my belief that Google interprets low click through as not answering the user's question. As a result, they may choose to show your links less.