What type of industry are you in? Be vague if you have to, but I would add to the conversation that you could be seeing a seasonal shift in search behavior. I have a site that saw a similar shift last month, but it was due to the main keywords being seasonally driven - looking at the clicks line that was far less volatile in the downward shift than the impressions tab.
In the screen you put up, was that for overall rankings or a specific keyword? I'd be interested to see if it was a shift in one keyword that did this. In one of the other screens there's a keyword that shows as generating 12,000 impressions, with the next one at 4,500. If you control for this single keyword how does the graph look?