Ok, so sadly this is normal. You got a little taste of YouTube virality but the market dried up... that happens with every video that goes viral. Some just last longer than others.
After you publish a video YouTube pushes it through several cycles of tests. They test you in a very large circle for the first hour to get some data on your viewership, and then they begin to hone down your targeted impressions to specific demographics for a few days, then another for a few days etc. From what I have seen there are multiple cycles that range as far as 140 days, but there are possibly more than that. Each circle gets smaller...
...unless you get a hit.
You got a hit where your video sparked something that caused YouTube to explode your impressions. It could have been a serious spike in CTR, it could have been a spike in watch time or you may have been riding the wake of a small trending topic (likely a combination of all these). YouTube was pushing you... and the more it did the less the results continued. That's when your video dropped off.
The good news is that you have some information to go on for what works. Break down your video data, see where it excelled and try to reproduce those results in future videos. And a little more good news; YouTube also has that data on your video. They saw something work... and they are going to continued to feed this video (and likely future videos) in this direction to see if it happens again. It could always bounce back in one of the future impression cycles. As a personal reference; I don't know why but I had a dozen videos all pop after 140 days. There was just something about that cycle that connected for me. No idea why... but it happened consistently for over two years.
Another thing to consider; don't do it yet (YouTube is still testing the waters with this video). But if things don't come back by the 90 day mark for this video it is one that I would ABSOLUTELY test with different thumbnails and titles. See if you can give it a little kickstart after YouTube has exhausted their more recent efforts.
Hope that helps... good luck and congratulations!!!