Let me just peer into my good old crystal ball...
...Hmm, I'm seeing your video on the traffic sources page in analytics now. I see a big blue spike, that then fades out.
Browse features. When you mouse over that spike, you see that the vast majority of your views/watch time came from "Browse features".
Was I right?
No crystal ball, it just sounds like your video got featured on the home page. You know, when you're on the front page of YouTube and you get suggested various new videos. That's what likely happened to your video. Those suggestions don't remain there forever. While it's on the home page you'll get a disproportionate amount of views, which is why getting videos featured there is instrumental in growing a channel quickly... but soon it'll no longer be on the home page and it's back to the old crawl of views from search/related videos.
It can end up back on the home page again later if a sudden large enough spike in watch time happens on the video to trigger the algorithm to pick it up - I've had a particular video end up on the home page for a lot of people at least twice now, the most recent time being triggered by a large YouTuber discussing the subject that my video is about, causing a mass influx of people searching for it. The added watch time made the algorithm pick up my video and start recommending it.