Please don't take this the wrong way, but I'm having a hard time imagining a 6 month old site getting that many search impressions without engaging in something at least a little nefarious.
The pattern actually reminds me of what happened to a test site of mine. I created millions of pages in a day with a combination of different product feeds. The site took off at first, but it didn't last long. Within a few weeks it disappeared entirely (as it should have).
Did you do any link building? Is the content original? What type of site is it (ecom? informational)? Are you using product feeds?